Murtonfan, it's simple logic. Theriot steals bases at times when it's easiest. If he were to force in 40+ more attempts, he would be running at times that are suboptimal conditions for stealing bases, thus he would get caught more. You can't simply take every player with over 15 SBs(or whatever arbitrary endpoint you want to use) and say, man if only he tried to steal every time he was on base, he'd be even more valuable. Stolen bases are a risk/reward thing, and you run when there is the least risk. You want him to run when there's more risk of being thrown out,(be it a pitcher better at holding runners, a catcher better at throwing them out, or for whatever reason when he can't get a good jump) while assuming that he'd keep up the same pace he is now.