+1 I have noticed how easily you can be defeated if you are fighting against a horde of enemies. Your party will get obliterated quickly and your like "What the hell just happened?" If you have a party of at least 6 members then it raises your chances of survival especially against tougher enemies or larger groups.
If nothing else, because 1- It gives more targets to the enemies to chose from, possibly even parting their damage more. 2- It gives you more chances to intervene in the queue of enemy actions instead of staying a passive witness.
Which is why "concerns" that a six-men party would "slow down the game" are mostly an uneducated nonsense.