I've ridden my MTB 100 miles (coast to coast), plus other events, and I'm predominantly a road cyclist.
I bought the MTB to help with weight loss, and it's done exceptionally well, and I'm already looking forward to my next bike (CX), but I have reasons to that purchase.
My MTB has a lock out front fork (which is permanently locked), plus slicks. But as others have said, it's about what you want. Are you happy with the work you're putting in currently?
If so, keep at it, if not, then that's your choice.
Edit: I also commute on a daily basis, plus various other rides and rambles, and I keep the same tyres on. Hope this helps?