This may not answer your question but 5 players in a party is a very ordinary team set-up found in most MMOs. It gives room for 1 tank, 1 healer and 3 damage dealers (dps). And even if there's only 4 vocations, some vocations can fill several roles in the party.
I can't answer as to why this is the magic number - but I assume it has a lot to do with balancing the bosses. Also, since there's more damage dealers than tanks or healers it also makes sense for there to be additional spots for that role.