To the OP: Sounds like you've found the rebound adjustment on your shock and cranked it down to where the shock has minimal effect. That's what I had to do on my older MTBs that didn't have a lockout. My current full suspension bike has lockouts for both front and rear shocks, and I'm glad to have them because I often ride miles of paved roads or logging roads to get to local trailheads. Even with the suspension locked out, though, the MTB is still much slower on the road than my road bikes.

I agree that suspension on a commuter bike is overkill.... wide tires can provide enough comfort for most people.