Well, Toby IS bait. He is stolen strictly to get under Sarah's skin. Jareth assumes the spoiled little brat won't succeed in getting the kid back and breaking the spell or whatever that has them both in the Goblin Kingdom in the first place. However, she starts inching towards maturity and well, that's that. Throughout the movie he's annoyed about her progress. However, we start to see a change about the time of the ballroom dream. Her will has stopped being completely annoying and has started to attract him. If Sarah rescues Toby, the spell breaks and he will lose his chance to win her over. Since she treats him like a creepy stalker (even though she fawns over him something terrible in her little play dramas in the neighborhood), yeah, that's not likely to happen. She wins. Game over. More solitude for Jareth. Though, at the end, Jareth is implied to rethink Toby's pawnhood.
Now, I know there are haters for RTL, but Mayor Spittledrum lets slip Jareth is not the first ruler of the Labyrinth (though this is contradicted by Mizumi's flashbacks). Somebody's making crap up. I guess we'll need that new prequel comic to get the real deal on Jareth's past.