Steve,
You're right, of course, about the spinnaker boats. You can break a mast by releasing the mainsheet. But I also should have clarified that I was talking about sailing upwind and on close to beam reaches. Even on my Wave I don't cleat the main unless it is really light air. Maybe this habit is a carryover from my monohull dinghy days.

However, the main should always be cleated going downwind, spinnaker or not.