I don't understand the point of that autoplug-continue handler. It looks like it says "yep, I want this" for certain formats, then it turns around and says "no, I don't want this after all, decode it again" in the pad-added signal. I'm sure that complexity exists for some reason, otherwise someone would've deleted it long ago, but...
It's recent. The main idea is that we want to know the compressed format, but we don't necessarily want to output it. This is mainly for WM and mfplat, which have an API switch to toggle that. For DirectShow there's no such API switch so it doesn't matter.
I'm not sure it's the best design (and I've been thinking about other options), but it's what we have for now.