It definitely can't be fun for someone like Zatanna or Dr. Fate to suddenly be powerless when Black Alice borrows their powers, especially if they might be in the middle of an important battle or something. Also, what must people think when they suddenly lose their power, only to have it inexplicably return later? I'm pretty sure Black Alice isn't well known enough for every magic-user to realize that she's at fault when their powers momentarily disappear.
Furthermore, in the case of someone like Etrigan the Demon, it must really suck to lose his powers for a short time. One minute Jason Blood might over-joyed, thinking that his curse is finally gone, only to have it return as soon as Black Alice doesn't need it anymore.
Kind of makes me wonder how certain magic-users actually survive without their powers for even a short time. "Bullets? Ha! My magic makes me immortal! ... Wait, is that my blood? I feel cold all of a sudden..."