Regarding Harry Potter:
The first two films were directed by Chris Columbus, and he was the only director in the series interested in participating in the making of extended editions of the films. That I suppose lends those extended editions a legitimacy that the others don’t - which may be why you can buy them in stores.
The other six films have “extended cuts” that were cobbled together by Warner Bros. for ABC Family… those edits aren’t terribly good, if I recall. People here could do a better job.
For ‘Azkaban’, the additional scenes are totally inconsequential, as nearly all of them revolve around a subplot which is completely nonsensical and not even fully represented in the deleted scenes. They’re best left on the cutting room floor.