Like it or not, but Lilo and Stitch kind of is considered a classic now and with how beloved he is by the Japanese, of course he's in the show.
Actually, Belle and Beast, Aurora and Phillip and Snow White and her Prince appear on the inflatable screens in that whole Princess medley sequence.
As for the Witch's arrival, we don't even have a translation of that scene, though it definitely looks like it's a "Mickey's messing with things he shouldn't" thing. (EDIT: Have
a post from MiceChat translating that confirms that this is the case. Mickey asks the Mirror who's the greatest Sorcerer and gets himself in trouble with the villains)
I gotta say most of these "It's just random crap unlike the original" arguments are mostly driven by nostalgia.