[RickB]

Like just about everything today, the external 1911 extractor is designed to cost less for the manufacturer, while, hopefully, providing at least the level of performance of the original design.
The advantage, if any, is that the external design requires no adjustment if it has been properly designed and executed. If the spring is creating too little tension, then a stiffer spring should be an easy fix (if S&W offers a selection of springs, and if not . . .).
The greatest disadvantage, in my mind, is the proprietary design, which applies to all external 1911 extractors; you can't just order a "1911" extractor.
I think Kimber was the first manufacturer to offer an external extractor, and a gunsmith said he had experience with six distinct versions, after which they dropped the external and went back to the original design.
SIG and S&W appear to be doing well with their designs.