Perhaps I should have been more specific. I don't care about mercy. But your big brute of a warrior, should be able to safely spar with the wet-behind-the-ears rookie without pummeling them into the dust. Sure, you're going to dodge their hits, but there is a thing in LARP called 'pulling your blows', where you don't thwack the monsters with full force, but go for 'touches' (because they are humans in goblin costumes, and bruise!).
Being able for the uber-warrior to be skilled enough to know how hard to hit someone...and to not go full-out on someone they know is just learning, would be wonderful for RP.
Limit skill gains (if you're 'training', don't get gains, if you're being trained, 75%?), but it allows you to emote, and RP around going "Tighten up that footwork rookie", and so on, rather than it be 5 rounds and done.