Not doing DPS when you have gaps is just plain lazy and pointless. What are you doing when nobody needs healing? Twiddling your thumbs? Sipping your coffee? I find it hilarious that you don't think Holy does significant DPS. Not counting crits with a Relic weapon you are doing 600-700 damage to each target plus stun on the first one. 3-4 Holy then recover mana and refresh heals and that's 2400-2800 damage not counting crits from just you. If you add damage from your DoT's in cleric stance it's even more significant damage. Ever DPS on Demon Wall? It's insanely fast if the healer does DPS before the bees appear. Or second boss in WP. Watch what Holy does to the adds XD. It's hilarious.

Why are you even worried about Mana? If you are smart enough you'll use Shroud or carry pots for that. Better yet having a Bard in the party makes it even easier to DPS. You are throwing excuses in to be lazy.