I can't remember if I previously commented in this thread. But I'd argue for four character parties because:

1. I don't want to manage six characters gear, skills, etc. It can already get pretty tedious with just four party members.
2. Combat can already be way too slow with just a four character party given that the game is turn based.
3. I actually like the idea of playing through the game multiple times with a new party, but admit that this is a personal preference and may not be best for the game.
4. If they make certain encounters dramatically different depending what party members are with you - it will be cool to experience the unique interactions, combat encounters, etc. (Example: Lohse from D:OS2)

To mrfuji3's point though - unless you play the game several times, there is definitely a fair amount of content you will be missing out on.