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When I wrote about Ryan Rowland-Smith's locked front leg, Kyle Boddy of Driveline Mechanics left this comment (italics were from my original post):
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“Another mechanical flaw that RRS possesses is his locked plant leg as he releases the ball. It’s a flaw that many pitchers have that’ll make you cringe if you watch it frame by frame. The problem with it is that it shoots a power surge back up through the body that scrubs much of the forward momentum and forces the pitcher to produce most of the velocity with his arm. As you can imagine, pitchers with this defect often fall victim to arm injuries, especially in the shoulder. Oh, and not to mention problems in the knee, hip and back.”
I disagree with this assessment. ASMI notes: “…[pitchers with] greater front knee stabilization and extension will throw with greater ball velocity.” Furthermore, joints are meant to handle directly in-line forces like extension and flexion but are NOT meant to handle side-to-side movement (hence ACL ruptures/tears occuring on mostly sideways displacement unless hyperextension is achieved).
Overall, I don’t much care if the pitcher “locks out” his glove leg. In sports science, extending the knee joint is the first step of an efficient push against the ground – we call it “triple extension.”
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I don't fully agree that the lock isn't a bad thing, but he's the biomechanics expert (relative to me, anyway).  What's interesting is his comment about the side-to-side movement, which probably applies to Snell when he had a crooked foot, but not to other pitchers who have the locked knee but a straight foot.