Another viewpoint into this is strengths and weaknesses of your party. If you have 6 members, you will have the right tool for everything in the game. However, if you have only 4 members, there might be some areas where you are lacking, you have weaknesses. This gives you an opportunity to be creative.
No healing? Well perhaps you need to stock lots of health potions, magical items or take another route and avoid taking any damage.
Nobody to search & disarm traps? Perhaps you need to make your tank capable of taking the hits then.
This means more replayability too, because if your first playthrough didn't have any arcane casters, perhaps quests and approaches you take are completely different than if you had one?
I don't call this replayability... I call this a boring and bad gamedesign that reduce my possibilities to force replayability.
Assuming you're playing a custom character, if I'm not wrong : 3 companions out of 10 is 120 possibilities. 5 companions out of 10 is 252 possibilities. Both numbers are important if you want to talk about replayability and choices.
Of course that numbers don't care about alignement and character's personnality/compatibility.
I hope they'll give an option for a team of 6 and I hope we'll have many "followers" that aren't animals or creatures...