People leaving after getting loot means you have to get a replacement, but does it really affect the party that much? In most cases I have seen, not really as not all alliances wait for everyone because they are impatient and you can kill the bosses without a full alliance anyway (unless the replacement is a healer and then most of the time people do actually pause until they reach the group).
I have also done many runs in ST where the whole party has passed on every item up to the last boss, leaving potentially the whole PT to lot against for oil or UAT if it drops. I don't really see it as any different to that. You win some and lose some... (in my case usually losing XD )