Basically if an add-on keeps its own records and refers to user information in its own tables, when a user is deleted, it may potentially cause database errors when the corresponding record is processed by the add-on. We have quite a lot of add-ons installed and not all add-on authors are careful enough to take such issues into account. Rather than spending our time on finding out the source of such problems and fixing them, we simply refrain from deleting users to avoid them in the first place, especially if it's a longtime member with lots of activity on the forum.