They're popular (sort of) with trap and skeet shooters, where the effects of recoil are cumulative. Even if it helps just a tiny bit, that tiny bit helps over a couple hundred rounds.
I put one in my single shot 10ga, but it replaced a big bar of steel in the stock. I then added some extra weight in the now excess space, the recoil reducer being shorter than the bar. Could I notice a difference? Maaaaaybe? Right then, when it was fresh in my mind perhaps.
But I think a 1 pound mercury reducer (just making up a number) will work just a tiny bit better than a 1 pound hunk of lead would.