Are there any mechanical benefits to opening the Sisters' hearts beyond what is needed to unlock all the chambers? Obviously doing so only requires opening all of the stationary Sisters first hearts and some of the second hearts. Once this is done, is there anything to be gained from unlocking the hearts of sister's I don't intend to "go all the way" (so to speak) with? I've started to work on opening the first two hearts of all the stationary Sisters and have even opened Echo's 1st heart (and what's the story with Aya? When the heck does she show up?), but I'm kind of wondering if doing so is just a waste of Color...

You get more info. Whether you consider this a waste of color or not is up to you.

Well, there's a reason I stressed mechanical benefits. I'd love to hear everything every sister has to say, but I wondered if I got anything concrete out of giving them Color once all the chambers are accessible. So it's definitely a waste of Color from a mechanical standpoint after that point, hmm? I'll have to keep that in mind. I'm not yet sure which Sister I would choose out of them, so...