Long Lost Friends is mostly a marketing scheme to draw AP holders/locals/regulars to the park. If the characters appearing are announced in advance, it may even convince people who are avid fans of certain characters and have to make a longer trip to get to the park.
Other than that, it can be a nice little perk to visitors who didn't even know it was happening and introduce kids to characters they might not be familiar with... therefore making them want to see the movie they're from.
But I'd say the main reasons characters are "retired" are casting, costume upkeep, and popularity.
Well everyone loved Vidia and Perry so why boot them off? Their not gona remove Pixie Hollow altogether are they? I know the Disney fairies films are done as of 2015's film.