Dave has ruined as many pitchers as he's helped. But nobody remembers them.
Dave has a system, and if you don't follow the system, you're put on 'probation'. If you continue to fight with him, you're either demoted or traded. That is what happened to Anthony Reyes and also to Jason Marquis (and many other promising young pitchers over the years). Dave Duncan's system relies on EVERY single pitcher throwing EXACTLY the same pitch - the sinker. He believes that as long as you're throwing sinkers and you have a good defensive infield, the contact will average out in your favor over the course of a game. I'm sure he has plenty of statistics that support his philosophy, otherwise he wouldn't be so devoted to it. Now here's the tricky part. I admit that it is a fairly good long term strategy. That's how the Cards have been able to sit at or near the top of the division for a decade. But some pitchers have their own natural 'stuff' and would be better served by throwing their strengths. If they try to do that, though, they get pulled from the rotation or sent down to Memphis. After toiling away in these lesser roles for extended periods of time, they eventually capitulate (if they ever want to pitch for the Cardinals again) and abandon their own stuff in favor of the sinker. But not everyone can throw a good sinker, not to mention it chews up the arm if thrown all the time (hence, all of the injuries to St. Louis pitchers). So, the ones who can't master it end up as damaged goods. Their reputations are tarnished (both their pitching reps as well as their personal reps after Duncan gets finished with them - again, Reyes and Marquis) and they never get a chance to succeed. Duncan's system works wonders with reclamation projects (aging veterans and guys coming back from surgeries), but if you're a rookie pitcher with a blazing 4-seamer and a back-breaking forkball, the last place you want to be is with the St. Louis Cardinals.
^The above was written mostly in reference to starting pitchers. It also applies to some of the relief staff, though.
*Furthermore, the 'all sinkers, all the time' philosophy also can be exploited by specific teams when they have the right hitters. Patient low ball hitters (the Troy Glaus type) can sit on sinkers and drive them out of the park just like a leisurely day of golf. When a team has several of these hitters, the Cardinals are in trouble.