They're making 110" screens now (maybe bigger). There are applications for huge screens, and huge screens need lots of pixels. Pushing the number of pixels up is a good thing, even if we can't think of an application right now.
E.g. It would be nice to be able to see 4 PIP displays on our 42" set. But even if the cable box let you do that, the pixel resolution at 1080p is too small to read text that way. E.g. a screen for the weather, a screen for Google News, a fish tank screen and a screen for a news conference or speech or something. Bloomberg clients would love something like that...
Yeah, we won't see 50" 4K screens for under $1k in the home any time soon. But I think it's coming.
There's more to life than movies. ;-)
Cheers,
Scott.