The idea that it can cause impaction is anecdotal, but knowledge of what garters eat in the wild could also be used as an argument against it. In the wild, they eat primarily furless prey (fish/worms/frogs) and maybe the occasional newborn mammal. Many don't ever get big enough to eat something the size of a hopper, especially if it was alive and struggling. Given this, there's no reason to think their systems were built to handle large amounts of fur.

Basically, even if there's only a slight risk of impaction from fur, I'd still feel safer feeding pinkies and knowing that there's a zero risk of impaction from fur.