Quote from Winged
Loot only drops for you if you're in a certain proximity of the creature that was killed. So if your friend is 2+ screens away and kills a boss, you won't receive any loot, or Exp for that matter.
So therein is another reason for even random parties to stick together, both for more loot drops and Exp gains from monsters killed by teammates in your proximity.
All and all I feel this is a nonissue due to the proximity concept.
I know for a fact that this isn't the whole story. In the videos the Yogscast made, Simon died on the skeletion king and got ported far away, as Lewis killed the boss. Simon could then go in later and pick up the loot. Here is the video: http://www.youtube.com/watch?v=FutNsZVjawo
This is probably an exception though. Although even if the proximity is one screen away or a little less, it can still be a problem as rare items could drop from a random barrel that you missed and that some guy behind you opened right after you passed it.
And overneathe, I see your point, but there has to be some way to come to a mutually beneficial solution to this, mutually being both kinds of players... Or maybe there isn't, maybe you need such a strong motivator as loot to help randoms stick together.... Merh, your point is a good one, I'm a bit torn on this matter now