If you have 20 agents mailing in the same area what happens...the law of diminishing returns....I know plenty of very good salesmen that went on to do other sales due to the BS quality of leads.
Also I think I should mention a friend by the last name of Simadoroff who worked the FE market running direct mail leads in the 80s and 90s. He had a very pronounced accent and was working the rural south.. He made and exceptional living for years running DM leads. This was in the day before GPS and he would go to the post office to get directions to the homes. Running leads in those days took a lot more work than today. But, for the agent who would work, leads worked.