I'm not 100% sure it matters when it comes to luck. I think a monster's total loot is rolled against luck one time and it rolls against the luck of whomever did the most damage. Dunno if that has anything to do with why everyone disbands... my guess is that they don't want to share the special drop chance with others. Or maybe more people get a chance at a special drop if they aren't all in one party? I really have no idea.
I'm going by a uoguide article (FYI) -
Luck - UOGuide, the Ultima Online Encyclopedia