When news broke in December 2010 that Microsoft was going to port Windows to ARM, I said it made no sense.

The argument back then was this: Windows on ARM would mean discarding the thing that makes Windows entrenched and important: Windows applications. Tablets need all-new applications, and if you're going to run all-new applications then you don't really need Windows.

The reason for embracing ARM was that ARM processors were fundamental to building tablets with low enough power consumption that they could last a full working day from a single charge. It was worth giving up everything that x86 compatibility provided to get that low power consumption.

It's now almost two years later. Windows on ARM has a name, Windows RT. Windows on ARM has hardware, including the Asus VivoTab RT and Microsoft's own Surface. We know what Windows RT is, and just as importantly, we know what it isn't. Does it make any sense now?

Let's look at the evidence. Windows RT is, in essence, the whole of Windows 8, ported to ARM, more or less unmodified. The same kernel, many of the same drivers, the same Explorer with the same desktop, the same Start screen, the same development model for Metro apps: this is one operating system that happens to run on two different architectures.

No half-featured port

This is a point that takes a while to sink in. Does Windows RT support multiple users? Yes, because Windows 8 does. Does Windows RT support Flash in Internet Explorer 10? Yes, because Windows 8 does. Can Windows RT run Internet Explorer 10 on the desktop as well as in Metro? Yes, because Windows 8 can. Does Windows RT have the same bundled applications, like Mail, Video, Music, Weather, and so on? Yes, because Windows 8 does. Does Windows RT support Bluetooth mice and keyboards, USB hubs? Yes, because Windows 8 does. As a general rule, if Windows 8 has a feature Windows RT has the same feature.

This is not to say that getting Windows running on ARM was a trivial undertaking. Typical ARM SoCs don't use PCI for their integrated peripherals or ATA for their mass storage, and so Windows had to be modified to not require PCI and to support booting from MMC storage. But those changes are now part of core Windows; Windows 8 systems built around Intel's Clover Trail platform will also use MMC.

There were limits to Microsoft's willingness to accommodate the many variations found in SoCs: instead, the company standardized on a few things (for example, all devices must use UEFI booting and ACPI for hardware discovery and configuration) with the result that Windows RT devices are a little more PC like, and a little less varied, than has traditionally been the case for ARM devices.

I said "more or less" unmodified. There are a few differences between Windows RT and Windows 8. Some of them are just the usual SKU differences that Microsoft enjoys creating. A lot of enterprise-oriented features are omitted (just as they are from the base Windows 8 SKU). Windows RT can't join a domain, be a Remote Desktop host, or boot from VHDs, for example. It also has a few omissions of its own. In particular, it lacks Windows Media Player and Windows Media Center.

Then there are the two major differences. First the big bonus feature: Windows RT comes with four Office 2013 applications. Word, Excel, PowerPoint, and OneNote all ship as an integral part of Windows RT. They're not quite full versions of those applications—they lack macros and certain other extensibility features—but they're very close. These applications run on the desktop, and although Microsoft has attempted to make those applications touch-capable, you'll really want a keyboard and pointing device to use them.

And the big missing feature: Windows RT can't run third-party desktop applications. At all. Office is the only desktop application allowed. The only third-party applications allowed on Surface will be Metro-style applications bought from the Windows Store.

In this way, Microsoft made the problem I alluded to much worse—developers can't even port their applications to run on ARM. At the same time, however, it also provided a platform for the creation of new, touch-friendly applications.

There is one final wrinkle. The Office apps that ship with Windows RT are not licensed for use in any commercial fashion, whether for profit or otherwise. In a practical sense it's hard to see how this would ever be enforced, and it's inevitable that someone somewhere will end up breaking the rules, even if unwittingly. In fact, I daresay it's already happened: I'm sure some of the reviews of the Asus VivoTab RT and Microsoft Surface were written in whole or in part on Word in Windows RT.

Commercial organizations wishing to use Windows RT in a BYOD (bring your own device) fashion can still do so, but they must purchase suitable licenses (for example, by switching to one of Office 365's corporate plans) to enable this. This is presuming that they don't need macros or add-ins to do their work.

So what about ARM's power advantage? That may still exist, but it's nowhere near as clear-cut as it was two years ago. Back then, Intel had nothing even vaguely comparable to the high-end ARM processors. But today it does, with its Medfield smartphone processor and Clover Trail tablet processor, both sold under its Atom brand.

In the time it has taken Microsoft to bring Windows on ARM to market, ARM's once overwhelming battery life advantage has been erased. The ARM CPUs may still have a slight power use edge, but the difference will typically be dwarfed by the power consumption of the screen. The Intel processors, in turn, bring CPU performance that is probably best in class (or close to it), and most importantly of all the ability to run the full version of Windows 8 and existing Windows applications. The hardware could look identical to the user, but if it has Intel inside, the user experience will be quite different.

Pricing still isn't known for all the Atom-powered Windows 8 PCs, but indications are that it won't be far off the price of the ARM machines.

This all puts Windows RT in a very awkward spot. The downsides I described in 2010 are a reality, making it hard to see what Windows RT devices will do better than comparable Windows 8 ones. Windows RT devices are beholden to the Windows Store, and while that may one day be fully populated, right now it isn't. Windows 8 systems can fill any functional gaps with legacy desktop applications; Windows RT ones can't.

Were it not for the licensing oddities, Windows RT devices might be useful as companion devices and in BYOD scenarios. Even if Atom machines cost the same as ARM ones, the presence of a "free" version of Office would give the ARM devices a useful advantage. But the licensing situation essentially eradicates the value of the bundled Office applications. To use them commercially requires a commercial license, and if you're buying a commercial license, you're better off with x86 Office—you get the full application suite (including Outlook), and you get the full set of functionality (including macros).

No clear purpose

With these constraints and limitations, it's hard to see who exactly Windows RT is for. I acknowledge that there are certainly some users who will be content to use the browser, mail app, and perhaps type the occasional letter in Word or balance their checkbook in Excel: people for whom the Windows Store's current gaps do not matter.

But I think a much wider selection of users will be ill-served by Windows RT. While I think it's likely that developers will produce Windows Store apps, I don't know how quickly they will do so, or how well: Google Play, the Android store, is still substantially bereft of high quality tablet applications, even with many millions of devices already in users' hands. Without a thriving store, Windows RT is just too limited to recommend.

ARM may end up being a little cheaper than Atom, but Atom ensures that your investment will not be diminished through a lack of software. Even if you're bullish on the Store and believe it will quickly be packed with desirable applications, going the Windows RT route forces a commitment to and a gamble on the Windows Store. Play it safe with Windows 8 instead.