and motoring with main-only. But I rarely see boats motoring with genoa-only. I think the main reason many people sail with genoa-only is because (with roller furling) they find it easier than hoisting the main.
When sailing downwind in strong winds we'll sometimes sail with genoa-only. Sometimes in light air (less than 5 knots) we motor with main up, hoping the wind will come up enough to sail at a decent speed and allow us to kill the engine, at which point we'll deploy the headsail. We almost never motor-sail with genoa-only. Why? Two reasons: More sail area and power in the main, and less visibility with the genoa.