Of course, that depends on when you finish it. You do have the funds to buy training at 25k a pop, and not worry about it. For the most part, though, you have everything you could possibly want, so the answer is no, there's no use.
I've had plenty of builds where I was still dirt poor by the time I got my last Stone. It all depends on your playstyle. Personally, despite the nature of the quest, I think they did a real good job implementing it. If it's approached as an afterthought once you have a bunch already, I agree, it's pretty pointless by then but if handled early and efficiently it can really bolster your build. When I know my build is gonna be leaning towards the TG, I knock that quest out as early as possible, get that Paragon displayed in the Cistern early so Mercer Frey knows his days are numbered.
A good way to kill two birds is to join the TG and get Bedlam jobs from Devin. When he sends you to a Hold for the job, steal a Stone. It satisfies the Bedlam requirements on it's own. So you can get paid 500 septims to steal the Stone and be one Stone closer to completing the quest.