Takes the right tool for the job! On that note, there are times when I purposely choose to commute to work on my MTB, when my route is only about 50% trail, and all-road on the way back. It's just a more comfortable bike to ride, plus I get to partake in a little "urban assault" with road furniture or short dirt patches adjacent to the road on the way home.

I also take a little bit of guilty pleasure in passing folks on road bikes while on my MTB on the road... I can get truckin' fairly quickly if I get into a low position and turn the screws a bit. Being a competitive fit shaved-legged type helps