The point with both Shield and Spell Shield is that they are very cheap to cast Mana-wise, and you can recast them as soon as they get low on hit points.
Spell Shield will also protect you from poison damage, AND stop those annoying little twitches poison causes your character to make.
They are cheap to cast, but they take a while to recast, so you really have to stumble away from what you're fighting to recast it, then, as you've spent so much time with your arms straight out casting, you'll have to retreat again in order to get into good spell casting range. For melee fighters, this is nearly useless; you spend so much time recasting it, it'll nearly be beaten down again by the time you're done casting.
You are quite right about its 1-hit life saver, though -- a careless mistep near a Giant Heavy Orc will kill you, but if you have a single point in Shield, you can limp away.
Restoration I do like because it's spammable -- simply right click a few times and you should be healed right up. However, if you're playing a mage, you'll undoubtedly be lower on mana than you are health, and should probably just chug a minor restoration.
