It does just seem to be a convenience issue, and I can't see it selling that well.
I can get BBC iPlayer, ITV Player, 4OD, and Demand 5 on pretty much any PC in the house. My Bluray player will pull in iPlayer and Demand 5, although for some reason doesn't do the other two.
A website or application that let me access all of these would be useful, but buying a box seems unnecessary. There seem to have been various issues with anti-competitiveness that have partially been the reason behind this sort of thing not being done. I'm not sure how YouView can do it, whereas they couldn't release things like Project Kangaroo in the past, but I guess it's in the niceties of exactly what's being offered up. Seesaw which appears to have also encompassed some of these aspects, including pay-per-view, tanked, but that seems to have been as much about investment and not managing to negotiate agreements between the parties involved, as anything else.