I don't like the way Verlander straightens the front leg. Obviously, he's very successful at it which just goes to show you can get good at doing things in many different ways with enough practice and enough strength and flexibility. But that's not a good reason to teach what Verlander does.
In general, I think straightening his leg creates for a more violent delivery. I prefer pitchers to look smooth and effortless as that is usually a sign of effective use of the body. Violent deliveries just "seem" more dangerous.
Specifically, I think he straightens his leg early and that affects his timing -preventing him from delaying shoulder rotation as long as it otherwise could be. That prevents him from getting as much out of hip and shoulder rotation as possible. He has to make up for that elsewhere - probably witih his arm.
The timing issue also keeps him from getting his release point as close to home plate as he otherwise could because, without delaying shoulder rotation, he spins out early instead of tracking forward.