I think "low" priority isn't appropriate.
Deleting a site from database cut many many mb! Just think about the visit_log, and shrinking visit_log leads to a optimized archive process, smaller archives tables, a quicker db, etc.
(In ) Fixes #585 Now, every entity linked to a site is deleted when a site is deleted.
Only the logs and archives are not deleted, because it could result in severe data loss. Better deal with this later on..