Excellent question.
Well, the 'more desirable' variants use the 'Model F' technology, which is the earlier version of buckling spring...as seen in the PC, XT and original AT keyboards...which use the same sort of spring mechanism, but the flippy hammer part at the bottom actuates a capacitive sensor instead. The difference in feel and sound is surprisingly significant.
Each key sends a unique scancode...this makes them very apt for remapping to do assorted things, even macro functions. Using software like AutoHotkey you get a lot of flexibility...you can do this with any input device, but having extra keys makes it more convenient. Whether you want to have a key to run the 'ls' command with a specific set of switches every time, or you want a key to open this website in your default browser, or media keys (without having to contend with a lame keyboard, as generally only lame ones have media keys included), the extra keys are helpful.
I've moved away from the hacks to use them over PS/2 natively (they do use the AT protocol, just a quirky implementation of it - possibly the earliest version, as they predate the AT itself by some period of time) and am now using a USB converter somewhat equivalent to
this one, but which has not been released yet (I'm somewhat of a tester for it).
The desire for this particular keyboard came from two things...wanting to make sure it behaves properly on the same converter code as the other boards, and also because it's a cool board...you get the extra keys, and regain some of your desk space. As I don't often need a numeric keypad it's an acceptable trade-off (if any substantial amount of numbers had to be entered, though, I'd definitely go to a board with the keypad).
