referential integrity only means that the db will not allow you to delete a parent record (sales person) if there are any children (leads)
If you force referential integrity, you need to be extra diligent in the way you craft the application.
You need to make sure that if you are going to try to delete a sales person, that you first delete any leads that have added by that sales person.
Do you _need_ to force referential integrity? No.
can you benefit from forcing referential integrity? Yes, it will help keep your database clean, and free of orphaned records.
There may be instances though where you want to keep the orphaned records.
As a developer, the choice to force referential integrity needs to be evaluated on a per application basis.
If you do choose to force referential integrity, you need to develop the application to account for it.
