I'm content with 4 companions, someone at Reddit summed it up nicely, though judging by responses there is lots of die hard fans of 6 members and no matter arguments everyone against them will hit a wall so why bother. 4 that's the number.
That's fun, I think the exact same about those saying that 4 is the number and that explain it with totally invalid arguments...
The only valuable argument I read for a strict limitation to 4 characters comes from players that really like limitations. They see it as a challenge because they have to choose "more wisely". Why not, I can hear that...
But everything else (probably 80%+) comes from totally invalid and thoughtless argument. (More variety with 4, slower combats, everything has to be balance again... If only it was atm...)