Not sure if this has been pointed out yet (probably >.>), but the tutorial said "disguise yourself as a FALLEN enemy". Which seems to imply that you have to kill someone, or at least find someone who has been killed, to disguise as that person. As opposed to TF2 where you could disguise yourself as any class when you wanted.
Yea, it would be stupid if you could take it from people still alive, they would notice it... It works the same as in previous SD games.