a "D" makes sense for the following reasons:
1) There are 4 people in light parties. One position for each person in a fight.
2) 4 roles in most raids (tank, healer, rDPS, mDPS) with different positions
3) a lot of bosses, such as shiva, use more than 3 locations for things.
4) Before a fight, when demonstrating a mechanic, multiple marks is helpful.

There's really no good reason to not have a D other than "4 is bad luck" (which it is in Japan. But we have 4 person parties....)