Well, there is the fact that they can coordinate instantly to focus down a single character very quickly, without having to type or even speak. That's an advantage whether it's 1v5 or 5v5. So it's a way to avoid having to (gasp!) cooporate with others in an mmo, which is a little cheap, honestly. But they're still easy enough to take down if they're not played well.
Shrug, that's no different to 5 players being on vent or skype (or well it is, in that you have to approach it a little differently, but I wouldn't call it harder to deal with per se).