That is a point I totally overlooked. There are probably more that I overlooked, but he isn't going to recharge himself often when he only transforms yellows to purple anyway. As another user mentioned in a post further up, allowing him to change any color to purple would make him feel more mythic.
I decided to give it more thought: he at least has huge synergy with Valk, her creating both souls and blue gems. Too bad he does single target damage AND wrecks yellow gems, though. Also too bad he totally overlaps colors with my beloved Khorvash.