If they're playing their part in the group and decide to use sange for whatever reason they desire and everything is going well, I don't see a problem. I don't think currently that it's useful as a primary attack, but I don't suspect that anybody would be using it as a primary attack, but they might view it as something useful when the enemy goes out of range as somebody has suggested. Mathematically it might not be the best thing to do, but it's one area where a person might wish to make use of their sange if they preferred and I would have no problem with that. If I'm grouped I want it to be successful but I also want people to have fun, if playing about in a style they prefer suits them and we still walk out successfully I wouldn't suggest they're doing a bad job and I am happy.

If how they're playing poses any problems, then I'd make suggestions on how they can improve so whatever we're doing is not a failiure. I personally wouldn't use sange myself, but then I prefer sticking to meleeing on my NIN and would rather use any funds on ammo for COR & RNG or on tools for NIN.