Many people would find them too slow, especially cheaper ones. Heavy, lots of energy-sucking pedal bob, and rolling resistance.

I mean, if you're *never* riding it offroad, it seems like one would get all the problems of a MTB with none of the benefits. Personally, I wouldn't want to ride a lot of road miles on a Boulder. But that's just me. If the OP loves it, more power to him. But I'd bet he'd never look at that Boulder again if he tried riding something that didn't take a significant effort to maintain 15mph on level roads.

As it happens, I am looking to pick up some sort of MTB that would be used substantially for commuting. However, that's because there are routes to work that would take me offroad. Even still, I'll only consider a MTB that has a fork lockout. I also already have a road bike, so the new MTB wouldn't be used for full-time on-road duty, and even still will probably be something of a compromise with skinnier-than-usual tires and a stiffer fork with less travel.

Basically, the typical MTB just doesn't seem like the best tool for the job if all your miles are ridden on roads. But by all means, do it if you like it - you'll certainly get a good workout!