Holy has a cast time of 2.5 seconds and a stun duration of 4 seconds (with DR leading to 4 > 2 > 1 > resist). You theoretically lose 1.5 seconds of stun if you chain cast Holy. Generally, Holy > Aero 3 > Holy > Holy is what people would do to maximize stun uptime if they cared enough about it. Since we don't have Aero 3 now, what are you doing between Holy casts 1 and 2? Nothing? Then you're failing your ABCs. Applying regen? You should already have it up during the pull, since you should be running alongside the tank. I guess now we can Holy > Misery > Holy > Holy, but we need a dumb lily readied to do that (and that only applies to level 74 and above).
WHM had absolutely zero issues casting Holy ever since Assize and Thin Air became a part of our kit. The cost is effectively meaningless for WHM until such a time as our mana economy changes.