To add to this, not everyone is so lucky as to have a pre-made 5 man ready to go when they log on for the night. Maybe they're guildless, maybe their guild is off in Naxxramas, maybe their guild is 10 levels below them, but whatever the reason, a lot of people end up looking for the pickup groups to get their dungeon diving done.
Anyhow, you probably know what happens next. Pickup group plus loot equals drama. You'd almost think that's some sort of universal constant in WoW. Sure, we've had some great PuGs, but we've had some bad ones too, and those bad ones seem to come complete with ready-made loot drama, and it seems like we're not alone. We're hearing from a lot of people lately who've had problems with loot distribution drama.