Oh fuck off. You must be the most unimaginative person to play with in the whole world if your don't get this.
First off, it's called Create Thrall for a reason. Why the hell dya think that is, huh?
(http://tvtropes.org/pmwiki/pmwiki.php/Main/MoreThanMindControl - it's this, basically)
Create Thrall doesn't *need* to mind control the target. Otherwise we would call it 'Dominate Person', which is shock of shocks, a spell that already exists in the game!
What it gives the player is the ability to speak directly into the mind of someone who permanently thinks well of them and will believe what that voice tells them. Anytime, anywhere, for as long as they want.
A persuasion roll with charm can't exactly do much. Hundreds of them sure can.
The point isn't to be 'Dominate Person', the point is to gain allies that you otherwise might not have had.
Say you use it to Thrall the captain of the city guard. No. He is not going to kill his own men for you, or kill the King. But he could be convinced to turn his eye away from your crimes, or he could be convinced to give you information you wouldn't have gotten, or tell you important secrets. Side with you on criminal cases etc. etc.
Or hell. Convince him your the voice of his god, or of a ghost haunting his mansion or something, and drive him totally freaking nuts at all hours of the night by screaming into his head so he can be replaced by the party member you've been working up the ranks.
I love features like this. Theyre fucking fun! Sure, it requires imagination on your end and you need a DM that will actually co-operate with you. But if you don't have those two things why the hell are you playing this fucking game?