I suppose the only real reason I tend to pass on mountain bikes these days is the lack of drop bars. I find the extra hand positions really help me on longer rides and they seem to help me make more power on climbs.

I tend to use my bikes for group rides as well, and a MTB wouldn't be well suited for that. On our rides, the few times someone brings a MTB they end up falling way back.

I think a good MTB would be perfect for doing heavy hauling with a trailer to a local destination though, because of their tendency to have better gearing for that sort of thing. I don't think I'd want to tow anything particularly heavy with either of my road bikes.
.
.
.