Also note that getting (non-stacking) attacks yourself is progressively worse in multiplayer games if the attacks have a limited benefit to you (they usually do since they're attacks).
I mean, if you played the second Militia of the round right after another guy, you're only hitting the guy who just played the first. Curses also tend to spread less dramatically. If you don't Curse along in a 2p game, it's going to be 10-0. If you don't join in in a 3p game, it's more likely 5-5-10 or 6-6-8. So you only have a 2:1 or 1.3:1 disadvantage instead of infinite:0.
2p is a 0 sum game, meaning: If I attack an opponent, the disadvantge to his deck is a direct advantage to mine. If I give my opponent a Curse, that's pointwise the same as gaining an Estate (without having a dead card in your deck of course). If you give one opponent the last Curse in a 3p game, the other opponent has gained an Estate as well without even doing anything.