I really hate when people precast regen and medica II (especially the latter). It's so pointless, it's not succor and aldo. Precasting Stoneskin is an entirely different matter though.

I stoneskin the DPS, I really don't see why you wouldn't other than laziness. Sure, it's 5 seconds longer you have to wait if I cast it at the start of a duty, but it saves me MP in the long run since they can essentially absorb one or two relatively large hits, it saves me from needing to heal them if they somehow manage to get hit by an AoE. It's essentially free mitigation when cast outside or near the very end of battle before everyone runs to the next set of mobs (because you know, you can estimate when the last mob will die and pop SS before it does). There's no reason not to use it on the DPS.

Popping stoneskin during in the middle of battle can be a large waste of MP though. I've seen several WHMs in duties popping SS on people when they really should be focused on healing.