I believe it,s more convienience than anything else......I,ve built several sidewinders(93") and used teflon buttons on all them,they all had forged pistons which usually are set up a little looser than cast.
You really want to check with the peaple that make the pistons your going to use!!!
use the style keeper and pin that goes with the piston. The teflon button is just a cheap way to machine pistons. That said, I've been running S&S with buttons on my engines for 30 years with no worries.
Ive also been running buttons longer than i care to say.With no probs.Ive worked on alot of cyl. over the with walked clips though.Also to me asembly easier and faster set jugs on bench upside down slide in pistons till almost to base turn over slide over rod slie in wrist. buttons and send him on down for just a lot easier,also I can be nice and carefull compared to trying to compress rings slide stuff down pain.Believe it or not you can slide rings in to bore on the bench with just you finger nails honest!