Knobby MTB tires make for crappy commuting on pavement. They slow you down and just aren't well suited to pavement.

That's the biggest down side to a MTB as a commuter. There are some others, but that's the biggest one in my opinion. (I did it for a year and bought a a hybrid for commuting)

You can mitigate that by getting smoother tires with less tread for the streets. Basically turn the MTB into a hybrid. And if you want to switch between on and off road stuff regularly get a second set of wheel for the street tries so you can quickly just swap out wheels based on what kind of riding you're doing that day.

This is a good option on a cheaper bike with cheaper wheels. On a pricier bike with nicer wheels that starts getting expensive in a hurry.