I don't see why mismatched tires would be a bad thing unless you're noticing any problems. If you're not losing traction in the rear with the slick then there is no reason to put knobbies in the rear.
I ride a single speed cyclocross for fun on streets to fire roads and I have a 38c Specialized Crossroads tire which features an inverted tread with a continuous center line for less rolling resistance and in the rear I have a 32c more cyclocross type tire with horizontal treads to help resist skidding out while mashing on the looser stuff.