STM is relevant for movies, not so much for still pictures. I don't know if 55-250 is a useful focal length for movies but from still pictures perspective, I'd rather have USM installed. It might drive a little bit the price up but from my experience using 55-250, USM feature will be very much welcome for the 55-250 for a little bit fast AF.
With its lightweight construction, the micromotor AF of the 55-250 really isn't that slow at all, so no need for USM, I would say.
So USM, no. STM? Possible but more for marketing purposes than anything else.
Sorry to ask but have you really used a 55-250? I am using one and I've used a 70-200 F4 USM L. AF speed is a lot different. Most of the time it means getting the shot or not. I used both for events. I don't really know from what perspective where you able to say that the micromotor AF of 55-250 is enough. I've covered a children indoor school-olympics and I've got a lot of misses using my 55-250. I have to borrow my friend's 70-200 to cope with the kids.
I had one and used it on a 400D. Of course it can't compete with a 70-200 L but then I think it's good enough given the cost of the lens.
5D3, 5D2, Sony α6000, S90 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: SY12 f/2, Ʃ19 & 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses