We already talked about it in some of the previous pages (or maybe it was in the other big thread about this same topic?) but I want to stress a point again: it's not just about having an effective battle formation, it's also about putting a lot of great loot to good use, so you feel properly rewarded for what you find.

A party of six allows you to build a party where more or less all the range of weapon and armor types are used and useful.
In a party of 4 you either force this differentiation, limiting your options for a party composition even more, OR you'll end up with a lot of loot feeling useless because, for instance, you have one single character that can use "martial" equipment, no one wearing a certain category of armor, etc.