Charm makes them feel peaceful around you, fall in love with you, do whatever you ask of them, whatever you think you want. When the power turns off, what if they don't stop wanting you? What if they don't know how to take no for an answer, or they're the sort who just fawn over you. What if they make trouble for the people you actually like?

What if you think you want them to help you, but they turn out to be no use while they're besotten by your powers?

Or, what if they're like me... and when your powers turn off they go completely psycho on you and try making you nail yourself to the nearest wall so they can play with you?

Just... be careful when you use it. It's an easy power to abuse.