Having separate code paths for "it's a press", "it's a release", "it's a drag" just turns into a nightmare when you actually use all 3 input events extensively. Remember that while the motion events of the drag might be the visually interesting part, for the most part the interesting stuff happens at the beginning and end of them - i.e. the press and release.

It becomes massively simpler conceptually (and even in the code) to treat any button press as a potential drag, and a simple click becomes just a drag-with-no-movement.