We talked about this in the live show we just filmed.
https://steempeak.com/steemmonsters/@monsterworld/group-discussion-about-steemmonsters-game-play

They are aware of this issue and will do things to avoid some of this.
From what I got, not all games will add XP, mostly certain big games/tournaments will have this effect (of adding XP)

They could make a very small fee that is slightly more than the value of that XP they'll be receiving from playing in the tournament. There are lots of ways to combat these issues which you brought up which are very real.

One example is that if it's 1xp for participating we know right now from the market that this is worth about 3.5-6 cents.

I say keep the discussion going because we need people saying how they think things could be exploited... so it doesn't happen.