You don't have to have slicks to ride the mtb bike on the road. I did many miles on my mtb bike on the road before I got a road bike a couple months ago. The drawback to the knobbies is the vibration, and I always felt the pavement wore the tires more quickly than the dirt. I have a mountain bike that I use as a commuter which has tires with minimal tread, and that is a good compromise. Those tires are durable, flat-resistant, and can go just about anywhere.