For everyone crying "gimmick", here are some practical uses for the screen I can think of off the top of my head:
1: in a game like Okami, you can draw the symbols on the touchscreen
2: in an FPS game, you can have a selection of guns on the screen and tap the one you want to switch to.
3: in an RTS game, you can make little routes planned or make notes on a map, or give commands.
4: in an RPG game, you can put certain spells or skill on the screen as a shortcut to fast-use.
5: in anything requiring numerical or alphabetical input, you can use the touchscreen.
6: in a turn-based strategy, it can be used to move units and select action and micro-manage
7: in a point-and-click game, it can be used to... well, point and click.
8: in an option menus, you can toggle certain features on and off.
9: in a character creation screen, the screen can be used for those little sliders.
10: in a sports game, you can set formations and select plays you frequently use.
11: in a party game, you can doodle on images on the big screen.
And this is from a sleep-deprived 18-year old. Imagine what a competent game designer could come up with. And if devs are smart, they can simply make a game for the console and IGNORE THE SCREEN IF IT ISN'T NEEDED. The controller itself is good enough for most AAA games these days. It's not like the Wii where there were little buttons to use and motion control was all but needed. The controller is a lot like modern controllers with plenty of buttons and the screen can simply be used to make things simpler.