They had them on typewriters, so why do computer keyboards have no keys for common fractions so we have to dig around in character maps or make lists of alt codes, both of which put me off bothering. I know office occasionally converts a 1/2 to a proper one, but that has only happened once, and only on the 2000 version, not XP. So why can't they simply use all the space they use to add new function keys we hardly use for a few simple fractions- even two would yield four with a shift which covers all the ones I need usually.
I agree that it would be useful if common fractions were directly available from the keyboard.
The easiest solution in Word, though, is to assign some shortcut keys. If you don't use the 'F' keys much (or at all) these are the obvious ones to choose (and you can easily place some stickers above them to remind you which fractions they'll produce). Otherwise, you'll need to use either an Alt or Ctrl combination.