I suppose that's easy if you have a shield in your right click ability bar, you can activate and reactivate quickly. Unfortuantley I don't do that, there's too many different shields I'd rater have them all on one bar so I can clearly pick on out. (I don't use the shield hotkey either as I don't know how the game determines which shield to use based on the situation, if it does at all) But fighting a water esper boss for example, it doesn't fare well when you can only fire a few shots before having to switch ability bars, deactivate, recharge, reactivate, and switch again for only a few shots again.
That's one thing I'd like to see the game do..... is DIFFERENTIATE the different shields a bit beyond just element. Give each one a secondary advantage, so players have more reason to choose between them.
Mostly, I just use the fire shield; all of the attacks in the game that REALLY give me the most trouble are fire based, and the shield is generally decent even at deflecting other attack types. I'm careful about mana usage, so even with my high-damage character (around 225 mana), I can use the shield effectively; generally I might turn it on for a second if a difficult to dodge (like, in narrow passages, or whatever) attack is coming my way. Generally, I wont attack when using the shield, unless I'm using my high-mana guy, who can hold the shield and get a few shots off without it breaking.
The shield is very useful against the seeker flares fired by dragons and the Overlord, and also REALLY good against those bloody fairies; an earth shield would probably be a bit more efficient against them, but the fire shield absorbs those attacks well enough.
And using a controller, I generally have the fire shield mapped to the same button most of the time (while the others change based on what active spells I'm using at the moment), so that makes it easy to use.