I've received this email a few times before when the flight looked like it was going to be full; I think they're trying to save overhead bin space, and maybe also make a little $$ since we'd have to pay for the checked bag on the return flight.

Personally, I never bite at these as I travel with only a carry-on if at all possible. For some reason, baggage claim at MCO adds like an hour to the end of the trip, and I'd rather be home by then. Other airports are probably a different experience.