Ah handling systems. I started out learning Derrett. No clue if it's more popular here (on the west coast) than APHS. I do know, however, when I started, yes, more people were doing Derrett. It seems more people are doing APHS or a combo of that with European style handling. HOWEVER, I also noticed how the courses (mostly AKC) has changed over the past year too.
I agree that a dog needs a solid foundation of something consistent. And then, even then, when you teach a new trick, say ketcheker-sp? (blind cross), you stil need to train it, so they understand what you're doing. In Derrett speak, that is a big no-no. You never, ever turn your back on your dog or allow your dog to blind cross you.
However, I will always remember what Daisy Peel said to us. She told us, I'm not Derrett, I can't run like Derrett and I don't have Derrett's dogs. So I don't follow any one system completely.
For myself, that hit home. I can't stay in front of my dog constantly, I'm not that fast. I started doing what I needed to do for myself and Koji (as in, start learning to handle him in anyway I can, that made sense, for us as a team).
And honestly, that's my advice to people. Find a system to learn foundation. But as you grow as a team, adjust what you need to, to make it work for you. I know with Koji, he's always been one of those dogs that do things differently and confuse me. LOL.
I started out hardcore Derrett. But now, I just learn whatever works for us. If Koji doesn't seem to get or like something new, we try it a different way. If that other way works well, I go with it.