Originally Posted by
Ambrodel
One of the disadvantages (and one reason you don't see many groups like that), is coordination. It is far easier for one or two people to take a food/toilet/telephone break and "be RIGHT back". In larger groups you often get slowed down by one person juuuust having to do this or that and it then takes longer than expected, so the remaining people either carry on too far and have to wait for someone to catch up, or they just sit and wait. And wait. Aaaaand wait.... And just before that person comes back, someone else needs to do something.
In a hardcore group, sure, you can avoid problems like that. But outside of the new-expansion rush, not many people bother with hardcore leveling. It's easier to just go solo or duo.