I think the trouble with the Fullbringers explanation of their motives is that there doesn't seem to be any reason why the Fullbringers should actively be trying to give up their powers as opposed to just never using them. They show no outwards signs of having powers so there's no real threat of ostracism and as far as we know there are no other adverse affects to having them and at least a few positive ones. Given that, why would a bunch of people willingly want to give up cool powers they have, and if they do want to give them up why don't any of them seem all that bothered by using them.

Case in point Riruka's goofy smirk any time she sucks Ichigo into her dollhouse or the blase way she interacts with the guy she kidnapped and pressed into service as her training dummy. She acts like using her power is as natural as breathing and she has such terrible emotional control it's hard to imagine she's putting up a front but secretly hates being able to abuse people indefinitely with no consequence.