More storage? Better keyboard? If you're going to be using it with a keyboard 90% of the time, an Ultrabook makes more sense to me really.
Surface Pro is already 128Gb SSD and a choice of touch or chiclet keyboard so hardly lacking in that area (although of course there are one or two Ultrabooks that offer 256Gb if that is really important to you).
You said
"They've said on a par with Ultrabooks so why wouldn't you buy an Ultrabook instead? Whilst many of the geeks and nerds (no offence, I'm one of them) on here like the look of such a device, it's very niche in my opinion and won't appeal to the mass market at all."
Assuming they are on a par, and in addition Surface Pro adds a choice of keyboard plus the touch functionality, I just don't think the average informed consumer in that particular market would prefer a bog standard Ultrabook as you infer unless it met a specific requirement (like the 256Gb SSD). This is particularly relevent as increasingly touch functionality will be regarded as a normal expected interface for a mobile device by consumers and the mass market.
Perhaps in direct opposites to you, I'd suggest the Surface Pro is no more
"very niche for geeks and nerds only" than a comparable Ultrabook and pretty much appeals to the same (growing) market. In fact it may well be that non touch enabled mobile devices become the "niche" offerings of the next five years, seeming incredibly archaic to the average consumer. To be honest I can easily imagine standard Ultrabooks going the same way Netbooks did when they had to suddenly compete with similarly priced and performant slate devices.
There may be a few "niche" scenarios where an ultrabook offers an advantage (again, perhaps a larger SSD as you suggest) but I suggest given the choice of a
identically speced and priced Surface Pro or standard Ultrabook my money is the vast majority taking the Surface Pro.
This of course is all in the perspective of the current and projected premium ultrabook market, which although growing is unlikely to ever be as mainstream as the $500 tablet market where iPad and Surface RT are likely to play.
Ultimately, to each their own I suppose...