It can be any pocket that's with you. But a 6" screen won't fit where a 7" can when the 6" is twice the thickness and has two controllers stuck on the side - the full system is about 10" by 4".
It's definitely not everyone. Of course, when I was advocating my Grand Vision of the Future with a portable gaming tablet/console, I was much more willing to believe people would.
I think if it's popular, yes, people will want it to be portable to share the gaming experience. If it's not, I think it adds little.