Yeah, Regen is an important, if not vital, part of a WHM's healing repertoire, however, you need to use it wisely. Because it's such a hate magnet, you must not cast it before the battle, but only after the tank has pulled the mob and got hate on himself. Then feel free to keep Regen up on him - it cuts down on spamming Cure/Cure II. Wise tanks will also remove Regen themselves if it's still active on them when they go to pull the next target.