Brake wear might be a consideration, break temperatures on constantly minimum time turnarounds however surely are. And the differences in speed by using higher flap settings make quite a difference. Of course the differences in variants is even bigger, so flying into a limiting field with a -700 is quite pleasant, with an -800 it can ruin your minimum turnaround.
I do not know why EU-OPS 737s are not allowed to used flaps 15 though, i'm kinda puzzled by that to be honest. Strange however that up to 25 for take off seems to be a rare thing too.