I don't want to hijack this thread but since POD has received the answer that Yoga is most definitely beneficial and many pro and college programs are doing it I will add just a bit more information on flexibility (and lifting to some extent)
As we all know pitching (actually all baseball for that matter) is about power and flexibility. While Yoga is a great first step to gaining flexibility there is a lot of Yoga that does not necessarily apply to baseball athletes, not that it won't hurt it just that it may be not the most efficient use of time. There is also a website called mobilitywod (link below) that is dedicated to mobility of athletes. Guys like them, eric cressey and P3 sports are taking the next step in understanding and implementing programs for athletes that are studying and developing routines that are even better than the accepted "olympic lifts", Yoga, etc, that are becoming more accepted in the training of baseball athletes.
By all means get into a Yoga class and start working on core and legs, as this is great stuff and will improve a pitcher, but just be aware there is more information out there and even better activities available for those who want to seek them out.