No as far as fitness goes you are right of course, I was having this discussion with Daniel yesterday who was saying I should get a MTB because you have to put more effort in so it'll get you fitter. But my point was that if you're exercise is limited by time rather than distance then it doesn't really matter if you have a MTB, hybrid or road bike as you are going to be putting in the same amount of effort over that time, but the distance you travel will be different.
But; I like traveling long distances, and there is a local cycling group most of which have road bikes that I'd like to participate in.
PS. Was out last night, all wrapped up, but it was still freezing :( . I don't think I'll be able to go out in the evenings much longer, so I'm restricted to weekends.