1 stop faster is nothing compared to 3-4 stops from IS. And before you say that the 1 stop advantage could be helpful with sports to freeze motion, the lighting needs to be so bad that your at the limit of acceptable ISO and can't afford to bump it up a stop and you're needing to shoot wide open. Image quality and USM are fair points, of course they both depend on what body Ultimate is using, and it's not as if the 55-250mm has ****e image quality to begin with.
You can buy a monopod to "add" IS to any lens. You can NOT add a stop of light.