OK, it's like this. If you throw the "traditional" way, the medial elbow & anterior shoulder structures are stressed & prone to injury. And if these things hurt, naturally switching to a different technique will bring you relief, possibly increase your MPH too because a lack of pain. BUT, if you throw this way long enough, other structures will be stressed. By the way it looks, probably the lateral elbow & posterior shoulder. The Marshall technique has not been used long enough, often enough, & used by enough practitioners to say it is better than the traditional way.
I'm not a big fan of the "my way is the only way- everyone else is ignorant" style. Marshall is not the only pitching guru who is guilty of this. Give me my high school coach, Ray Korn, any day. 1 elbow injury in 27 years, & not a single shoulder as far as I can remember. As soon as an athlete complains about something hurting, he has them do something different with their foot, or how high they lift their knee or something like that and no more pain.
Something to think about: Say your a cave man a few million years ago and you're starving to death and you see a woolly mammoth cross your path & you've got a spear in your hand. What throwing technique would YOUR body choose?