it probably would be very problematic for me to have holy power on all specs. although i would only play holy pally, i would have an interest to switch to protection to see how it played ( legion and BFA player ), but even without the holy power aspect of it, it was a bit complicated / unappealing for me.
and of course with ret, impossible for me to play. i remember i tried ret in BGs a few times and ended up playing in a awkward / perplexed manner. i basically couldn’t manage myself in mobility, healing, etc, and only knew how to go as far in a certain dps rotation which i didn’t even know what to press often, and in some occasions, completely wouldn’t even press anything at all. protection was difficult as well.
so to answer on having holy power specifically for which classes, then i would say only for ret, and keep holy and protection without holy power . i play holy paladin because it allows me to have a straightforward rotation and playstyle, do dps here and there while also healing. as a matter of fact, that’s mainly the reason why i play one in world PVP and BGs in particular is so i can play more like a off healer / pocket healer in a way and try to dps more when possible.
with holy power, it would make holy paladin too complex to play for me. for some of course, they would like how it adds this complexity or whatever. but for me, my playstyle consist of having my hotkeys and abilities set up in my abilities bar in a certain way where i can connect them together as i am simultaneously looking at the battle and buttons.
the issue with holy power aspect is that it takes to much capacity for me to know what’s to be used when and where in my screen and at the same time, press that ability efficiently. not to mention that i don’t use extra abilities bars. i only use the left or middle bar, and the added abilities would require me to add an additional bar either stretched in the middle or a right bar, which will make it even more confusing and distressing to play.
i’m already factoring in a few new abilities i may need to add, regardless of whether they are holy power spenders or not. so i’m basically trying to fit all abilities on the bottom left bar only. for example, i might need to get rid of a manual trinket buff ( instead using stat stick / automatic trinket buffs ) and replace that spot with a new ability such as the covenant ability vanquisher’s hammer.