User blog comment:B214/Volume 22 spoilers/@comment-82.48.158.172-20161109231053/@comment-30289101-20161112222427

That's a potentially valid argument, but far from a certainty. We've no idea exactly how much Rias' growth since meeting Issei has effected the power of her pieces, nor do we know how much the Sepheroth Graal "costs" respective to the Boosted gear. Not to mention, Plot, for better or worse, overrides any and all established rules in a story, and from a narrative stand point, it would make sence. Valerie would be replacing Rias' lost healer and has a strong connection to Gasper, her would be fellow Bishop. Intoducing a new character to fullfill a roll that's already easily covered by an existing and otherwise underused character is poor writing. Doubly so when that character is already fullfilling that roll and has yet to recieve a reason to stop.