I use my MTB for running errands and just for fun. I used to use slicks when I knew I would be on pavement more than dirt. But now I just keep the same tires on all the time, maybe use a little more tire pressure on pavement. I have some Bontrager Jones ACX on right now. The raised middle knob tracks well and is smooth on pavement.

I think the MTB is better on the body on pavement. The fat tires absorb shock better than skinny one. If you have suspension it might sap a bit of power on the uphills, but rewards you on the DHs. I also "FEEL" more secure with a backpack or loaded rack on the fat tires. Nothing scientific, just a feel.

I think what is doing is great. I just rode a century on skinny tires. Trained for a month on the road bike as well. I enjoyed it, but this morning when I was back on fat tires, I remembered why I prefer them. Smooths the pavement out. You feel every little bump on skinny tires pumped up to 115.