Theres rubber mats and then theres rubber mats. I heard a few crap turntables once didn't mean that I thought they were all crap.
The problems with matts is that felt is thought to sound one of the best. It causes too much static though which is a real pain. Apparently felt mats absorb all audio frequencies more or less equally, making them quite netural.
Other matts tend not to do this so you just have to keep trying until you get something you like that doesn't cause static. Sometimes a mix of 2 or more mats can work.
I've been hunting for a pure carbon fibre matt as they are conductive, no static but haven't found one yet.
Earthing the spindle is a very good idea.