The friction hitch on a tether, SRT, or rappel rope is holding all your weight, on a linesmen’s or adjustable bridge the hitch only sees half, because the end of the rope is also attached to the saddle.
The main job of a backup hitch is to grab the rope if the device fails for some reason, but if you can’t use the device for some reason are you going to be able to descend on the backup hitch alone?
All friction hitches should grab reliably when loaded, with no slipping. Some hitches bind up and need to be unweighted and loosened up before they will slide on the rope. Other hitches will slide without having to loosen/massage them, but still need to be unloaded first. Another group of hitches will release under load when used for a linesmen’s, or adj. bridge, when only getting half the weight. And only a few will release under load, repeatedly, under control, on a fixed/stationary rope.