To be honest, I don't think that PIE is that important, even as WHM... I play WHM as my main class, and I occasionnaly switch to SCH when the PF requires one, or when I don't feel like healing and let Eos/Selene do the work for me (only for low-gear dungeons).
I have to say that the more I play SCH, the better I play WHM, and vice-versa. Playing WHM teaches me to be very careful with my MP, my aggro (regen + medicaII when adds pop, and I'm dead) and when I switch to SCH, I always have 85-90% of my MP pool, because I am very conservative.

PIE is great, but not necessary, really. If you 'waste' your MP casting Raise ONCE, it's fine, and you should never run out of mana because of this. If you have to raise more than that, this is NOT your problem (unless you let someone die, which is another story... Sometimes, a player is better dead than consuming his healers's MP...)