It’s helpful when the bridgeplate has been damaged - e.g. by slotted pins allowing ball-ends to ‘creep’ into the pin holes - but not something I’d recommend if the bridge-plate is in decent condition.

I used that technique for a short while about twenty years ago when a Martin I owned at the time had exactly that problem - the cheap ‘n’ nasty soft plastic slotted pins that CFM used had distorted and allowed the ball ends to push them rearwards in the straight-drilled holes, and the ball-ends ‘crept’ into the holes. But, when I learned about bridge reaming and slotting and got advice from a couple of Martin experts on the UMGF, I reamed the pin holes with a 5° reamer to make the pins seat properly down to the collar, and slotted the holes. Once that was done, and I replaced the original pins with Antique Acoustics Galalith (a very hard plastic made from milk) unslotted pins, there were no further problems.

I don’t like anything that can interfere with the transmission of string-vibrations to the guitar’s soundboard (which is why I won’t have under-saddle pickups in my guitars, and use K&K under-soundboard pickups instead) and, to my way of thinking, an important part of the transmission chain is the contact between the ball-end and bridgeplate. Putting a second ball on the string is adding a barrier, however slight, to that transmission chain.

That’s my take on it anyway - I can’t offer any ‘proof’, it’s just ‘logical’ to my mind. Others may differ and that’s fine by me - ‘opinions are like arses’, etc. etc. ;-)