I expect that the major driving force for the change was cost. Another member recently posted an article that dated back to the 50's and one image showed Lapping of the barrels after the rifling had been cut. If they are still lapping barrels by hand for cut rifling I can assure that is NOT an inexpensive procedure. In addition when you machine steel by a cutting action, bad things do happen if a chip welds to the cutting edge or when the cutter gets dull.
A secondary benefit is that I believe that the new style of rifling provides a velocity gain with jacketed bullets, however I have no way of confirming that. One thing that is readily visible is that the new style rifling produces a surface finish that appears glass smooth. BTW, it would be interesting to see a comparison of the newer rifling against the older method, however variations between guns would mean that a rather large statistical study would be needed to insure reasonably reliable results.
As for shooting lead bullets in these new barrels, I used 50 rounds of Blazer 158 Gn. LRN in my 620 just once. That created such a mess that I won't ever make that mistake again. I've seen some posts that indicate the leading issues can be solved, however I believe that it's a matter of determining what and how much lube to use. Perhaps someone who is reloading for these barrels will chime in and give you some guidance.
As for accuracy, I've recently shot the 620 with a scope mounted and cannot complain at all about it's accuracy, in fact it's distinctly brag worthy.
