I agree with the OP actually. I can't speak for SCH since I'm a full time pvp whm but standing all day inside the circle, rushing into the circle when there are 8 enemies- especially as WHM is plain dumb. It's just way better to stand outside the circle which allows you to easily healbomb without using any CD's. 1 or 2 persons harassing you ? Np fluid aura, repose, sure/swiftcast repose and thats a few more seconds to heal without interruption. Or just LoS them and a 2 second cast ist almost granted. It's easier to find time outside the circle as when you're getting bombed inside the circle because that's exactly what happens. The circle is a mine field. No more no less.
I don't care what others say, if I can only heal for roughly 20 seconds in a 5 minute fight (and of those precious 20 seconds about 15 might get lost due to stuns and silence) I'm nothing but a failure. The only time I head to mid is when I judge that it is safe to go there and plan to use a few CD's. sacred prism, equan, healbomb a few seconds and then I'm out again. I'll stay in if I see it's safe but I'll run if it's not.

It depends alot on the Situation but generally you stay outside because you don't want to get interrupted all the time. VIT accesories won't stop the interruptions. It doesn't matter if I have 3800 HP and can't cast or if I have 4500 HP and can't cast. Those extra HP won't buy me enough time for my CD's to refresh. It might give enough time for the other healer to heal me but let's just assume a PLD is on that healer and he's unable to cast as well.