Despite the perception that he may have had a fine rookie season by putting up good scoring numbers last year, there's quite a few red flags to his game. But this being an audition and the fact that Landry wouldn't have lasted that much longer with us, it's a decent deal. Reke's out for the next three weeks, and while Jermaine Taylor might also deserve a shot with his one-trick scoring, Thornton's more proven in this area.
Thornton's best asset is his ability to create shots for himself at a high rate. He's more all-around scorer than shooter--he can get to the basket and jacks up threes at a good rate, all without turning the ball over. But he gives vibes as being a me-first player--he almost never passes the ball (despite hogging it so much), doesn't contribute much in other areas and plays poor defense. Despite that he was still a fringe starter last year, but if his offense collapses he has nothing else to turn to, and it can get ugly. The ballhogging is the biggest red flag--his style of offense (see below) is not good enough to justify it, and he can have problems making an impact if he's not scoring, so there might be no middle ground with him. He needs to hog the ball and score, or he needs to be benchwarmer.
Another thing--I think Thornton's near the tip of his upside already--traditionally players with low turnover rates don't tend to improve as much in subsequent reasons, and Thornton really takes care of the ball, for better and for worse in this respect. Also, he's only a one-area player--that of scoring, so the only area where he can improve is his scoring efficiency, but he appears to be just a above average (not elite) three point shooter and a below average finisher, and even though he slashes at a decent rate he's extremely poor at drawing fouls, so he isn't a real natural offensive player. He's just got good scoring instincts and can create shots easily for himself at will, but that's relatively typical for guards of his ilk. In fact it could run of the mill type stuff, and he could go the way of a dime-a-dozen undersized shooting guard chucker if his work ethic/lack of development isn't up to snuff (a la Flip Murray/Rashad McCants, and that might be why he was in Monty Williams' doghouse this season).