The question becomes, "Do I forego casting shield so my PoM has free run?" or, "Do I only shield people that are topped off so that PoM won't jump to them?"
The answer is, well... both. PoM is nice that it is fire and forget, but if you are seeing it heal once and then sit on a particular fight, you might want to reconsider where you start it at. Normally, the MT is the optimal primary target. A PW:S/PoM is a great way to put 12-17k worth of future healing on the tank. On fights like Marrowgar, or any similar fight that multiple tanks are taking constant damage, PoM really shines. Very rarely is it still active by the time the cd comes around. The shield is only active on them for one or two hits, and then PoM takes over and allows you to do maintenance. (And since PoM jumps are not throttled by a gcd, your healing spikes tremendously during this exchange.)
Other fights, it is not so dandy. On fights where the tanks trade off, like Saurfang, PoM will pop once and sit. I have found that more times than not, when PoM comes off cd, it is still sitting on the second tank. I have started to put PoM on the Bloodboil target with a shield, and that seems to have better results. After it jumps from the target to another damaged person in the vicinity, it has been moving to where the action is most of the times rather than getting one heal on the tank and sitting on another member who may or may not take damage. If possible, the person that receives the first Mark should move within range (20 yds) of the tank so they may share the bouncy ball of healiness. (Can't run to the cannons anymore anyway.
![Smiley: bah](http://zam.zamimg.com/i/smilies/bah.gif)
My point is that while both of these spells are powerful in their own right, changing your tactics to match the fight will allow these strengths to be in harmony rather in contention.
I was bored... or drunk... or bored of being drunk. Cocktail, anyone?