\ ... And dnd is written for parties of 3 to 4 players. You can have bigger parties but it quikly muddies the waters with regards to balance. It very easily becomes to easy or to hard if you muck that up. So giving us an option to do it would work imho. Maybe make it be tied to a higher difficulty or something or just place a warning that the game might be easier then it was intended to be. But more options for the player is rarely a bad thing and I couldnt totally stand behind the option of allowing 6 man parties.
I've heard that repeatedly (the 4-party 'ideal'). I assume that's because CR was originally based on a group of 4, and it involves some work for a DM to rescale encounters in P&P. But that really doesn't mean its intended for 4 players only. I have always played with larger groups. Yes, 5E is more versatile with class roles, so you can get away with fewer players, but really that by no means precludes or limits player numbers. I have also DM'd and recently played with 6 players in a 5E campaign. It is absolutely doable, so I really don't think that's a reason not to support more than 4 players. They could quite easily target a 6-person party and have everything 'scaled' - or as others have argued, if the XP awards are split, then the party level would self balance (or could ne made to) anyway and over-levelling on core content should not be an issue.
I would prefer of 6 (or at least 5) person party. I find 4 too limiting, even if I can cover core roles with 4. It seems like an artificial optimization exercise. I would like my ranger, cleric, wizard, paladin, rogue etc. as I did before.