Some pitchers have a leg motion like you describe (e.g. Nolan Ryan) and some don't. I don't instruct the leg motion. To me, it's personal style and, as JD pointed out, it's part of a pitcher's timing.
If the instructor you're referring to is teaching leg motion, I would disagree with that. If he's teaching the same leg motion to every pitcher, that's "canned instruction" which is inapproptiate IMHO.