I once had a combo micro-convection oven, but it broke and I never got it fixed, so I tossed it in the dumpster.
But you ARE supposed to be able to use the two features together as well. The speed of microwaving to cook the food faster, combined with the browning qualities and juiciness provided by the convection feature.
I got a separate dome-lid convection oven that works as fast as a microwave if not faster. It uses a powerful halogen heating element combined with a cyclonic fan.