I've experienced this a couple of times just recently.
It's understandable during game to have a character "go dormant" for a couple of minutes while the player has to do something else but I've been in Garden ops games just recently where, as you said, the Host is playing a Peashooter or another character that can climb up into some inaccessible area far away from the Garden or Graveyard which was spawned and they sit there the entire game. I've also seen it done by non-Hosts but that definitely requires that they do some little thing every now and again to avoid timing out on the server; I've even been in one game where two people were sitting it out far across the map just waiting for the evacuation phase.
On Normal difficulty I don't mind so much because that just means more vanquishes for me, but when you're playing Hard or Crazy difficulty having the game spawn enemies for 4 players while only 3 are actively defending is pretty annoying.
PopCap should come up with something that will prevent people from doing this, and yet give some incentive for people to continue hosting a game to the end even if they've been inactive for several minutes and exceeded the inactive timer.
