There are many fun combos, the problem lies in that nobody has ever valued fun higher than efficiency.
MNK/RDM gets interesting enspell effects, but people will just say "Lol, did you read the brady's guide or something? That combo is useless!" because you are better off being MNK/WAR.
and because of MP and skill and all that, but god forbid mellowy posts something that isn't misleading or wrong.
it's not the case at all that people avoid unconventional subs because they're "fun but not efficient". it's that most "fun" subs (say, anything/SMN so you can have a pet help you) don't work, and that's not fun.
as for valuing efficiency over fun for group activities, that's also not what's going on... it's valuing helping others over being selfish.