actually, they do wear down, which is good.
and since they are harder, they aren't as smooth because they don't absorb as many small cracks/rocks/bumps/ect.
you get more feel of the ground your riding on
just got back from atlanta yesterday, got a ton of clips downtown, the spots down there are on another level, and there are crackheads everywhere, totally worth the 2 plane rides and 16 hour car ride back, really sick trip