I know of one thing at -12v is used for.
Try playing some music (mod, mp3 or whatever)
The sound will probably sound terrible, the -12v is needed for the audio system, I don't know if its used for anything else as an Amiga WILL power up and run stuff without it (Hmm thinking about it, it MIGHT be used for the comms ports in some way or other, but not 100% sure on that)
If you dont need sound or comms, you might get away with it