Is bard charm worthless, or am I doing something wrong?

[Jakob]

So I leveled to 27 and bought charm. I keep charming mobs and it will only last for around 5 seconds until my song just ends. Then, if I try to recharm the mob, I can't, because he will still be charmed for a few seconds more making him immune to my song....

Am I doing something wrong, or is my charm useless?

P.S. I just thought about possibly charming one mob, then when the song ends, charming the hostile mob, and trading back and forth between the two mobs. Would that even work?