BARCELONA, MOBILE WORLD CONGRESS 2012 — On the surface of things, Asus’s Eee Pad Transformer Prime is just a wonderfully swell idea. Why have both an ultrabook and tablet when you can have one that is both? What if you could take your iPad, snap it onto a keyboard + trackpad, and have a MacBook Air?

It’s a nice dream, and, in actuality, the Transformer Prime is a beautiful piece of hardware. But the challenges aren’t hardware: they lie in software. And in software, neither Android nor iOS is yet up to the challenge of driving both a mobile and laptop OS. But after Windows 8 sets the bar higher, they both could be.

At Mobile World Congress in Barcelona, we had some hands-on time with the Tegra 3 powered Transformer Prime. Hardware wise, it’s a gorgeous piece of kit: a 1.3 GHz quad-core processor, 1GB of RAM, a new 1920×1200 display, and so on. Reported battery life is pretty good at 12 hours, but becomes sick when you attach the docking station, jumping to 18 hours. And it’s this docking station that gives the Transformer Prime tablet it’s other functionality: that of an ultrabook, with a full-sie QWERTY keyboard, a trackpad, a USB 2.0 port and an SD card slot.

So far, so good, and we can confirm that the Transformer Prime feels great to play with both docked and as a tablet. But as a laptop, it’s a total failure. Ice Cream Sandwich just isn’t up to snuff.

What does your laptop have that your iPad doesn’t? Don’t focus on the apps: you can find most of the apps you need, or their analogs, on any platform. With modern mobile operating systems, it’s also not multitasking. The thing that really distinguishes your laptop from your tablet is that your laptop can run two apps side-by-side, while your tablet can’t.

As a blogger, this is why I still bring my MacBook Air on the road, and not just my iPad. I need the abilty to consult pictures, videos or text while I write. And while you may not be a blogger, chances are, the reasons you still use a PC have little to do with performance or apps, and everything to do with running more than one app on the same screen at the same time.

Ice Cream Sandwich doesn’t let you do this yet. Nor does iOS. But when Windows 8 debuts later this year, it’ll allow you to run two apps side-by-side, even on ARM.

That’s just huge. Where the iPad — even with keyboard attachmenet — has always failed me as a writer, the Windows 8 tablet will actually excel. It’s the killer feature of Windows 8, as far as I’m concerned, and no one’s talking about it.

The Transformer Prime is a great piece of kit, and it does innovative things with merging keyboard with tablet that Apple’s not doing. Take the working trackpad: Apple has given us the ability to connect Bluetooth keyboards to iPhones and iPads, but not to use mice. Given how vocally Apple is against the so-called Gorilla Arm problem with multitouch Macs, it’s a strange omission that the Transformer Prime manages to correct.

But as long as mobile operating systems can’t run apps side-by-side, tablets won’t be more productive than laptops. How long will it take for iOS and Android to copy Windows 8’s best trick and catch up?