Here's a trick for you - the post length limit only applies to original posts. You can edit a post to make it as long as you want.
Anyway... Last night I had a tank in Sohm Al who sprinted away when I started casting Protect and grabbed 3 packs of stuff on the way. By time I caught him (because my sprint can't close the gap with him also sprinting), he was at 10% health. Regen isn't gonna get it done at that point. Hell, Cure 2 isn't gonna get it done at that point, since he'll be long dead by time it goes off. So hey, Benediction it is! Course, then the DPS started taking damage (because of all the poison flying around when you combine those packs), and if I stop tank healing long enough to help the DPS, the tank dies. That's exactly what happened.
There's no preparing for that, except maybe a macro on Protect saying "I'm casting Protect, don't run away."
I also think you grossly underestimate the variance in squishiness between tanks. I've gotten the same class in the same dungeon and it's night and day. One guy can be healed by Regen with an occasional other spell, and the other one requires my full attention. I've never played a MMO with wider variances in just how difficult tanks are to keep alive. Like, this:
Lovely advice, when you don't get some guy who thinks Sword Oath is just as good for tanking 9 things as Shield Oath. When you get that guy, you've got about 4 seconds before you're the new tank. Those are the ones that bug me. If I can't reapply Regen because the tank is taking so much damage that the GCD not providing immediate healing is going to result in death, the tank pulling too much stuff for their gear/skill. That's just a raw "HPS keep up with incoming DPS" race. I mean, I've done plenty of dungeons where I only needed Regen and oGCD stuff too. Those ones are AWESOME. It's the ones where that results in a swift and terrible death that we're talking about.Largesse > regen > Earthly star on tank > Collective unconscious on tank > observe tank health for next 15 seconds > respond accordingly with oGCD heal or using stellar detonation