WAIT - maybe I'm thinking of something else. Is it possible that they disallowed the ability for, say, +3 to Fury (Barbarian) and +5 to Arcane Power (Wizard) on the same item? That must be it, because you get zero benefit from those, at least the primary stats are doing something.
Now, that I could see, because that would be pretty much a waste if you're not that class.
There is no rule that all rare items have to be perfectly itemized. The bad ones make you appreciate the good ones. Melt the crap, wear the good stuff. If you don't like what you have, go kill more mobs.