@ PWilson
I'm not quite sure I follow your logic about stoneskin.
It's not a skill you spam, it lasts 30 mins for a reason.
You see the value of preemptive stuff, and you see the value on the non tanks, but don't see the value on the tank, who arguably gets the most benefit.

I guess I'd ask ya like this:
Proper MP management, protect, healing cooldowns, healer dps, etc. None are necessary at lower tiers, but can aid in picking up the slack / making up for mistakes made by newer players in pugs. What are you casting in the last 2.5 to 3 seconds of combat that is more beneficial than stoneskin?