Personal anxiety is definitely a part of it. I've talked with PUGs on card usage and personal tastes on a job per job basis to try to find some sort of happy medium, and I do feel trying to spread everything resulted from "well, this isn't going to hurt anyone and everyone gets buffed."
In the end I came to the conclusion that I'd rather contribute to overall DPS by just DPSing. XD That's rather counter-intuitive to the AST kit, so I jumped back on SCH at SB's launch. Currently leveling WHM.
But if you are looking for an explanation as to why so much spread happens in 4-mans, nerves is my answer