My first sealed bearings were in the Bullseye derailleur pulleys. The advantage was mostly that I didn't have to mess around with them, which was a big improvement over the plain bushings on the SunTour pulleys. This was important for the upper pulley on the SunTour Cyclone GT derailleur, which took some significant work to remove the pulley.

some quick photos...





As for headsets... is the modern A9 built with sealed bearings?
I'm only familiar with the old/original version with roller bearings.. which is a really robust design!

Steve in Peoria