I own a mountain bike in addition to the touring bike. I tend to use the MTB for commuting only in the spring, when roads here are gritty and can have a winter's worth of garbage collected near the curb. The MTB (a Mongoose Rockadile, w aluminum frame and front suspension) also has a gel seat, which is nice in the early season when I'm getting broken in.

As soon as conditions improve though, the geometry of the road bike makes for a much nicer ride.

In an Ideal world, I would own three or more bikes, to account for differences in terrain, surface, and objective.