Me and my brother play exclusive in 2-man locked games when we are hunting things.
The idea that letting strangers join, knowing it would punish us, is not something we entertain.
I'd love to run with 4-man teams but all it does is lower my chances of getting anything.
If me or my brother find two of whatever we're hunting then we give one to the other so that we both have something. I can't imagine the randoms that join us would be willing to part with their drops in the same way. The only thing we lose is the social aspect.
I'm more than happy to run 4-man teams when grinding exp but