Locking the brakes makes little difference, because as the vertical load increases, so the centre of the contact area moves forward of the axle, exerting a moment which tends to slow or stop wheel rotation.
Another reason not to apply the brakes is that rocking up onto a beach with brakes locked at just 20 knots could be embarrassing.
The last reason, which perhaps only applies to Husky pilots

is in an NTSB report: one of the aforementioned shaggy dog drivers set his parking brake for some waterskiing, then flew home and went ass over tit cos he forgot to release it. I know, we all use a landing checklist/mnemonic, but there but for the grace of God...
Incidentally, my Maule has a parking brake just next to the carb heat. I think it's the devil's spawn. Does anyone know the FAA legality of removing it?