I guess I would use a lead sled if I was an injured, an invalid, or was simply afraid of my rifle or shotgun's recoil. Otherwise I think it is a crutch and would have enough drawbacks to prevent me from using one. I believe that it changes the natural movement and vibration that are created when shoulder fired so that it would be quite possible to obtain false results.
It just might have a good use for velocity testing of ammunition. It would probably be the only way that a small person like my wife could fire some of my heavy recoiling rifles, if she ever had that desire.
To each their own. My choice (under normal conditions) is to say "No" to lead sleds.