I've commuted on mountain bikes. Only a few times did I commute on knobby tires. Like you said, they can be a blast to ride mostly due to their versatility.

Since all of my commutes are on asphalt both of my MTB's have slicks. The hardtail has Specialized Fatboys and the rigid has Specialized Nimbus.

Also remember, some people may not be that savvy and know slicks are available for mountain bikes, or their commutes are so short it doesn't matter.