On the show only reason why some can be used by others is because they have became a death scythe so they have no true meister anymore. Aka Why moka's father was used by like three different people through out the show. But it is clear that one in training can't just be used by anyone besides their meister and maybe another weapon partner (Kid had two who could use each other if his not around) because Black star and Soul teamed up to fight Kid but black star didn't use Soul for some reason so soul just tried to get in advantage moments and change to partial weapon/full weapon to strike when he could.
Now It has been years sense I watched/read it so could of been them being idiots, or maybe they said something I don't remember exactly.