*The* bishop? The one I'm thinking of?
Not sure.
Which is awesome and "cinematic"? The enemy managed to turn the tides of battle with a well placed backstab? And isn't that what various protective spells and trusted comrades are for? I mean, you don't just put your mage in the middle, tell him to commence casting diabolical spells, and forget about him, do you?
Well, a mage is not a Death Star, so it's not a single saving throw, that's the point. I agree that a single throw should not decide the outcome of a fight, but there are many different ways to deal with it and going with the Protoss' shields isn't the best one.
A mage should be protected, both by spells, which will take time and reduce his offensive power, and party members, not by convoluted mechanics aimed to prevent him from dying prematurely.
The coin toss you speak of is the direct result of a failure on the player's part. One of the best fights in IWD2 was the fight for the holy avenger. Keeping your mages alive there was a real challenge.
A very specific example. Just as you enter planar prison in BG2 you being attacked by bounty hunters, one of which is invisible rogue with Boots of Speed and +3 dagger on him. He runs directly to the weakest of your mages and stabs him.
Your position is determined by the game as you exit portal. Your front is too wide to cover and there is a way to circle you around from the back anyway, so fighters aren't of much help. You could cast glitterdust directly at the rogue, or hide in a resilent sphere or sanctuary (in case of mage/cleric), none of which you have any reason to do since you don't know if there's a rogue out there, especially considering that you didn't face threat of rogues up until this point in the game. You could also try casting protection from magical weapons (from the scroll, your level isn't high enough to remember this spell), which again makes no sense to do unless you know exactly what's coming at you. Finally you could mirror self, which is a theoretically plausible reaction (although, again, not warranted by the situation), yet this still means that your life and death are up to a chance.
Realistically speaking, all the effective solutions for this particular situation lie in the area of metagaming. But even if it wasn't the case, that's not quite my point. What I'm talking about is not that there's no way to succeed, but that success or failure are too volatile, and too much volatility is a bad thing. Let me illustrate my point.
Let's say you play a typical FPS, except that this particular one places one land mine somewhere on each level. The mine can be detected by a small hump on the ground if you look very closely. If you step on it you instantly die - game over.
Now, does it make impossible to complete the level? No. Even if it's incredibly difficult to spot the mine, it's your fault if you can't, isn't it? The enemy managed to turn the tides of battle with a well placed land mine. And you don't just run around the map carelessly when you know there can be a land mine somewhere, do you? All of the arguments you used above are applicable in this case. But would you consider such a mechanic a good idea?