I made the switch in Oct from a MTB to a road bike, and I am really glad I did. The biggest advantage was the different gearing allows me to continue pedaling at higher speeds than I could before. With the MTB, my top speeds were typically around 22mph and I was spinning out in the top gear. With the road bike, I would hit 25mph in the same areas and still be able to keep my cadence at a reasonable level. The road bike has helped me to work on having a better cadence in general.

Along with this, I enjoy going faster, and this also allows me to go farther. Most of my exercise time is time constrained, so the faster I go, the farther I can go. I still use the MTB with the kids, but I much prefer riding the road bike.