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?
That's all well and good until your party takes its first casualty. Then things get a lot worse real fast when the party takes multiple casualties. There were more than a few times that despite massive buffing from both spells and potions that four or five of my characters were down or out of the fight in IWD, BG, and BG2 and the only reason the party survived was because I had six instead of only four. No amount of creativity can completely make up for losing the only character with critical skills and/or abilities. The biggest advantage of having a party of six is being creative with the redundancies in your party so no single loss, and likely not even a loss of two, cripples the party. Losing the pure Cleric or pure Magic User doesn't hurt nearly as bad when the party has a dual class M/C for backup. Losing the pure tank doesn't hurt nearly as bad when there is another Fighter or Paladin or Ranger for backup. Robbing us of a perfectly valid tactical option is just spiteful.
If the traps are that wimpy what is the point of even bothering with traps? Making rogues irrelevant is just spiteful. One of my two favorite characters to run is a rogue as a scout/sniper/pathfinder. So if one of my two favorite character types is unnecessary why should I bother with the game?
Actually, a party of six provides for MORE replayability because a larger party makes more party configurations possible. So that is MORE opportunities to see what works well enough to succeed by trying different party configurations. Then people can try it with five instead of six if they think they've exhausted all of the six party configurations they have an interest in trying.
