IMOpinion, there's that ugly word again
, There are a couple reasons the metal bbs wouldn't be a good choice.
First, they will create wear on the rubber on the inside of the tire. There will be no evidence of this on the outside of the tire, but could result in a thinning of the rubber in one area, especially the area opposite the heavy area of the tire being balanced, that could cause a severe blowout with the accompanying damage.
Secondly, they will work as a "heat sink". They won't be hot enough to melt, but again they can damage the tire from the inside with no visible damage and result in the same results I mentioned before.
Third, they don't cake, but may take longer to achieve a balance in the tire than some type of non-metalic balancing beads.
Not sure that Tony's trucking company friend was using metalic beads or non-metalic. May just be nomenclature??