My phone's running 2.3, but it's got touch-sensitive areas under the glass to represent the system buttons, and I hit 'em by accident all the damn time. Add some extra and very much undesirable challenge to Fruit Ninja.
Hell, I occasionally manage to hit the button on my iPad Mini and back out of the book I'm reading or whatever, so I can imagine how much more frustrating a Nexus 7 would be. We've got several 4.x devices where I work, and as far as complaints about the OS from a user's perspective, those buttons are at the top of the list for the guys who have to use them often.
It doesn't help that Android apps tend to be even worse than iOS apps about consistently recovering their state when you briefly pop out of them like that, either.