Only reason for keeping the old version is if you're some kind of collector who wants to have many versions of the same game (it's understandable if changes between the versions are huge or if it's a complete remake). However in this case Special Edition is more like a patch. There's nothing really major. SE changes how the menus appear and it has new extras. AFAIK differences in actual game are improved graphics in some scenes and new alternative ending.
The major reason to keep the original is if any content was edited or removed for the new edition. I can't verify one way or the other because I haven't played through either yet.
I haven't checked, but I read a SE of something which added extra languages but also deleted support for some languages as well (due to licensing/distribution issues on the old translations).
Not certain if that was this game or another one,,,,