1 point of shield can defend you from a crunch power eg: hammer slam or fireball.

It is probably the most important power to get good with on Oak, as it is the difference of winning or losing a confrontation.

I find it's hard to time shield cast to avoid an incoming slam or fireball, but it's pretty damn convenient when someone on your team is getting focus-fired.  

Some of my uses for Shield 3:

(1) Go up to a portal tower on cataract (usually with monks and siege, maybe spirits), start hammering at it.  If someone TPs in, keep hacking at it until you're almost dead, then shield+tp out.  Come back when shield/tp cooldown is almost done.  Can usually down a portal tower in one or two goes.  If you're lucky, the whole team will TP in to deal with you, putting their TPs on cooldown and leaving the other lane empty.

(2) To relock a portal: TP in, shield, lock, surge of faith, hit the wand of speed and run away.  Likewise, when locking own portal when can't hold it: shield + lock, cannot be interrupted.

(3) Obviously, also can do things that would be otherwise suicidal, like going for enemy portals before the portal towers are down.

Use yout keyboard to trigger the skill so you can cast it in less then 1/2 a second, you can get many of the skills blocked if you are waiting for it.