There are several issues which affect when or how much the skipper cleats the main.

Along with the varying courses, water conditions, and wind strengths, there are some fixed differences.

Different skipper sizes and strengths.
A bigger/stronger skipper would be able to play(grind) more often than a weaker one.

Different size mainsails.
A Hobie 16 is easier to play than a Hobie 20.

Different mainsheet purchases.
A 10:1 is easier to play than an 8:1, with the same size main.

So an average strength sailor who plays the 10:1 mainsheet on an A Class would probably have to do alot of cleating on a 7:1 Hobie 20.

Ideally, in flat water and steady breeze, you could set the downhaul, outhaul, mast rotation, traveler and cleat the main and just steer the boat.

The only reasons not to cleat are needing to work(pump, grind) the main for more speed in gusty, shifty conditions and fear of flipping.