I have to agree that I prefer when people ask me to leave because how am I supposed to know if a PUG game (that has some open slots) is actually full or even a hosted game for that matter? Also, there are many without guilds, and good friends are hard to come by. If I join a hosted game, I usually do ask if there's room (but sometimes I'm booted with a letter before I can even ask or the level even loads!), but if it's a PUG game I don't consider it rude to join because it doesn't seem presumptuous to join an open game