I swapped my folding bike to a 29er mountain bike for my commute. The primary motive was comfort: I wanted the plushest tires possible under me. The 29er makes me 1-2mph faster on the same trip. It hardly feels like a difference on my 10 mile trip. I haven't changed my alarm clock. Comfort is king on my commuter bike; I wouldn't swap this bike for anything else because it's so damn nice to ride.

Unless you're really getting the maximum out of your bike, arriving all depleted and sweaty, I don't think it's a good investment to change the bike for anything. And a hybrid is going to be pretty close to your mtb anyway if you ask me, so that'd definitely be money down the drain.

Short summary of my opinion: if you haul ass all the time, get road bike. If you drive calmly, keep mtb. Don't get hybrid.