If you grind aluminium with a stone wheel, the aluminium will leave shards on the wheel. These shards will heat up with further use of the wheel, causing the wheel to slowly become fragile. A stone wheel spinning at over 5000rpm then suddenly breaking while your standing a foot away, you do the math. Aluminium isn't the only thing you shouldn't grind on a stone wheel, anything that doesn't make a spark will cause the same problems. I'm surprised you don't know this Andrew.
This is the grinder I have in my shed. One side is the stone wheel (caked with aluminium as you can see), and the other side has a belt and disc grinder. Both of these have a sandpaper-like surface, only they are alot stronger. The results from these are somewhat better than what you would get by using a stone wheel.