That very much depends on the add-on. The data for add-ons usually remains intact when you remove one, but often becomes invisible or unusable.
For Gliffy, that works by attaching two files to a page, a gliffy coded file and an image to display that's built from the coded file. If you remove the add-on, both files remain attached, but you won't be able to read or edit the coded file. The image will still display.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.