Deleting users is something you will want to think long and hard about before you do it. Whatever your reasons for doing it. it is important to remember that all of the consequences that come from deleting a user are permanent. Once you delete a user, that record is gone, you cannot get it back by simply adding them again. 

Pro:

The main reason for deleting user records is to unclutter the database. If a member moves away or expresses that they do not want to be contacted by the organization, you may want to consider removing them from the database.

Con:

The problem with removing a user from the database is that it deletes any record of that user. This can orphan any transactions connected to that user id. Meaning, that any ecommerce done by that user will no longer have a user id attached to it. Not only will ecommerce records be orphaned, but any forum posts or articles submitted will no longer have an author attached. 

Because of the permanent nature of this, we almost always recommend that no user records be deleted.