I ride my MTB on the road sometimes it's ok. The longest trip I've done on is the New River rail to trail ~ 56 miles one way did round trip in two days. That being said I also ride my touring bike on dirt roads. It's a Bianchi Vlope modified with the large gear in back 34 and an adjustiable stem. It has 32mm tires. Defenitly seems better for long rides no it's not as fast on super smooth pavement, but there is very little of that around here anyway and the 32mm tires seem good on rough pavement with loose stuff on it and of course dirt/gravel roads.

Ride what you like.

Joe