What about a touring bike? (particularly a sport tourer) Definitely designed to carry stuff, and you still get many of the features of the road bike.

I think the only advantage of a mountain bike is cheapness. If you aren't going to use it at all for off-roading then I would find a rigid one (ie no suspension) and change the tires to something slick, narrower, and higher pressure. Basically a DIY hybrid. In terms of a hybrid that already exists, what about something like the Kona Dew? Retails for $400, has 700c wheels but straight bars. Seems like a decent enough commuter.