As far as HUDs go, yes certain things are best kept on the main screen, like the pilot example, because often that info is designed to interact directly with the image on the main screen. However when it comes to separate things like inventory of course its easier to have it on a second touch screen. You're trying to make this big issue about looking away from the screen down at your hands, don't your arms move then? Can't you just bring the pad up closer to the screen? Having the inventory on your controller at all times means whenever you want to change items you only have to glance away from the screen, move your thumb a few inches and press directly on what you want, something that will take a second at most. A fraction of the time it'll take to bring up an inventory screen on your main screen and cycle through items with a stick to find the one you want. Not to mention there'd be no need to pause the game either. I think I know why you're making such a big deal about it though...
I know on my PS3 if I want to know what time it is I can either hit the PS button, wait for the Xmedia bar to come up and the clock to come onscreen, then hit the PS button again and wait for the game to come back, OR just look behind me at a clock.
I look behind me. It's a lot faster. Imagine now that all I had to do is look at that big ole screen in my hands. I don't see it being a problem at all.
I do think there are some things that will be better off on the TV, though. Things that are looked at so often and repeatedly that having them on the TV makes sense. I think the majority of HUD elements don't fit this bill.