Actually its bugged in two ways I believe. My crews seem to only be able to crit repair vehicles they just hopped out of (for whatever reason), making orphan crews only useful to repair to 100% (or have Thompsons )
I only try to save crews with vet, since vet is the only way you will be able to keep up with German lategame mediums and heavies.
Its actually bugged in several ways sometimes it plain just does not work i'm not sure why, but also it can be canceled if any other vehicle or something like that (i've had my own sherman do this) makes the crew move after you start it then it does not work. I've also had it not work for no good reason at all but put it on CD for my vehicle crew. It needs work for sure. As far as I know though you have to wait until the repair image goes away or it will take your munitons put it on CD and will NOT work, which is kind of varied in times if there is shit in the way when they hop out some times it takes much longer cause the crew kinda bumbles around.
Currently though I agree with Kaitof USF are way to munition starved atm to raise the cost at all. Plus it is not always that easy to do you can easily have something go wrong and end up giving your sherman to the enemy and really bite you in the ass. Most the time you also have to use smoke from the sherman too to cover it while emergency repairing which cost more munitions.