The xp penalty in my opinion is WAY over hyped. If you don't have a tank and a big fat Ogre SK shows up he is in. Or even a uglier Troll SK shows up guess what, He is in. You are not going to sit there for a hour waiting for one of 20 Warriors in the game to show up.
[You must be logged in to view images. Log in or Register.] By then the group would break up. And if it is a Guildie you sure as hell am not going to turn them down.
I know it does make a difference XP wise but hell it is just part of the game.