Is it best practice to require a letter from a spouse's employer when an employee LEAVES their company's health care plan for their spouses? I can understand requiring one from the spouse's employer if they are being put on a health plan

Is it best practice to require a letter from a spouse's employer when an employee LEAVES their company's health care plan for their spouses? I can understand requiring one from the spouse's employer if they are being put on a health plan when their employer does not provide benefits, but not when some one is leaving and going to a spouse's plan.