Just be glad druids don't have to expend valuable resources like pallies for their active regen. Hopefully that will change. Healing down-time (ie time where healing is not extremely intensive) is something you can usually find during fights (especially once u really get to know them), so its almost not even an hps loss to channel an active mana regen during those times, I do it on my monk with mana tea (b/c there's very little healing that would have been done during that time anyways), making any mana regen ability without a resource requirement very strong.
I'm not sure if they will change the holy shock numbers, but for some perspective as if this change were implemented now, let's look at Holy Shock: Holy Pally's most mana efficient HP generator. It costs 8% base mana which is 1.6% of your total mana with a 6 sec CD. So it takes 18 sec and 4.8% mana to cast divine plea, doing it the most mana efficient way. That yields a gain of only 2.2% mana and what makes it even worse, is that you don't get to use that 3 HP on a big powerful heal. A heal that would likely help prevent you from spending well over 2.2% mana on a future heal, which you now have to cast a regular spell and not a HP spell on. Use anything besides holy shock to generate even 1 holy power, and divine plea will not recover the mana used to get the 3 HP, and that's not even considering the big 3 HP heal that you now don't get to cast either. I could never see using divine plea over an eternal flame in its current state. Even if just for the mastery shield and hot, it would be better than a measly 2.2% mana. This has to be tuned so that the net mana gained (2.2% currently) is worth significantly more than an eternal flame (who's effectiveness changes with gear, meaing this spell gets less and less valueble the more geared you are), which it just isn't at all currently, especially considering that its only 2.2% if done with 3 consecutive holy powers over 18 sec, it usually won't even be that. Tuning the numbers could make it more viable, but it just doesn't make sense to me when it would be so much easier to balance/less problematic to just make the holy paladin active regen like the other classes and tie it to denounce.
Even if its tuned to be a mana gain, it's still a lot less effective than the other classes' active regen model ie: If a pally goes oom, he's in real trouble, a lot more than a class who can start casting a mana regen spell for free when oom, whereas a pally has to wait for the mana to cast the HP generators, to build up the 3 HP to cast divine plea, and also can't as effectively take advantage of downtime in the fight to regen man, but instead sacrifices GCDs and throughput during the fight to regen; meaning divine plea would have to be tuned even higher to be viable. Just doesn't make sense to make pally's active regen so different and so much harder to tune. I'm not saying it can't be tuned right, but I really don't see it happening and I don't see the point. Another factor to consider is that the value of this spell will scale significantly with gear, making it even harder to tune. As one's gear gets better, so will the value of an eternal flame cast, making divine plea less and less valuable relative to the alternative. Especially seeing as the WoD divine plea was originally implemented at 4% mana gain, a .8% net loss even if cast with all holy shocks, and then bumped a measly 3% to 7% they don't seem to understand the value of 3 HP and the non-resource tied mana regen they're giving every other healing class, and the difficultly of balancing this active regen compared to the others. Random imbalance in a key ability like this towards 1 specific class/spec generally leads to ppl flocking away from that class/spec. It's not worth the high risk of improper balance to give pallies a radically different form of active regen from the rest.