A lot - Frequently when I am living on my boat I don't have a car. My MTB (Specialized Stumpjumper MX) gets a rack and panniers and wheels with road tires. The MTB then does the grocery shopping, boat part shopping, and bar hopping. When it is time to go dirt riding I remove the road stuff, put on wheels with good dirt tires and am off to the hills.
I also have a nice CF road bike but it can't get the groceries for me.