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I think the psyker rules in general are pretty weak. The IG ones suck, and even things like Chaos are not as fun because it steals flexibility. A lash prince is just a lash prince. He can't call upon his Emperor's powers in his time of need. He just knows how to fzorgle.
Now, if you give them Eldar type powers which are useful and varied, you are almost right back to the Faith points. You would still have some flexibility, but at what cost? Should Sister's "faith" be just another form of psyker, the very beings they hunt down? What would seperate them from the varied beings that use the warp to fuel their abilities?

There is definitely a point where streamlining and making everything uniform starts to take away of the special nature of the army itself. Faith is currently the only thing making Sisters not "failed marines". Especially in a codex that includes SM units, taking that away from them really just makes them "Storm Troopers with mamaries."