Why the emphasis on mobile computing? Below the sheen of easy money or quick exit, especially for today's near zero capital expense, no entry barrier ecosystem, thanks in part to Cloud based systems, there is a truth in user adoption numbers. Take a look at this comparison of PC Vs. Smartphone shipments forecast (source:www.statista.com) and you realize why the VCs check for mobility access.
It is not just VCs stating a sort of minimum requirement, there are so many startups in this space with the big daddy of the rapid rises, Instagram who are fundamentally altering user behaviour that biggies are sitting up. You would agree that it is more than just a photo painting and sharing app. It is not just VCs, but developers and startups, and now a plethora of cross platform tools as demonstrated elegantly in a recent VisionMobile report (www.visionmobile.com)
The question really is not whether a minimum stated requirement i.e. to support a range of mobile devices is a triviality, but more a relevant user browsing behaviour change that is underway. So, 'That App' represents more than a specific app or a segment....and an entire behaviour. You remember those weird sci-fi looking bluetooth headsets for handsfree mobile in the early days, with people having frantic conversations with thin air? We are used to that now!