SysRq: Tied to PRTSC, if it ain't broken, don't fix it;
Scroll Lock: X-Fire uses it by default, and it's a great key to use when you need to bind a menu or overlay to something out of the way;
Pause/Break: I use the command prompt and command-line programs a lot. This key is the coolest thing ever.
Windows Key: Win+D all day long, Win+E very often, Win+R like it's a candy dispenser;
Menu Key: Incredibly useful when you can only use one hand to control your computer (especially if it happens to be your keyboard hand)
Function Keys: F1 for documentation, F2 to change names, F3 to pause, Alt+F4 for magic, F5 to refresh slashdot every 3 seconds, F6 to access the URL bar, F 7 when you're too lazy to use your mouse (or can only use your keyboard hand), FN+F8 to toggle wireless on my laptop, F9 to start recording with fraps, F10 to stop, F11 to take a screenshot with fraps, F12 is useful in a handful of apps;
So yeah, even if you're not using these keys yourself, they're very useful to anyone who actually uses their computers for more than browsing facebook.
Start coding anything with more than a few input commands and you'll actually be wanting extra keys