I’ve had this thought as well. That being said, being a jack of all trades, master of none can sometimes make it a bit more difficult in the focused HM/4* content where you can plan on covering your one big hole, rather than a few minor ones. Also, WP completely lacks DDR.
On the flip side though, it is one of the few sets that provides terrorize protection. I think it has a tougher time solo, but (theoretically) I imagine it could be an absolute monster with the right team behind it for the HM stuff. When you have barriers going off and people are buffing your defense and resists to the gills, I imagine Willpower’s HUGE regen would allow it to shine.