As another OG, glassing was not common at all and is a relatively new thing. Everyone flew cardboard unless you had money and could afford a Dynacom or Dangerous Dave fg kit. There was always PML with phenolic tubing as well but it was expensive compared to Loc; just below the midrange between Loc and the aforementioned fg kit suppliers. I never owned a PML kit, never had the money.
That being said, it was not uncommon though to do fg laminate reinforcing of centering ring to MMT/fin attachments as well as glassing the initial root pass of the fin to airframe intersection. Glassing the INTERIOR of couplers was also a thing as this was a more common failure mode for higher powered flights using cardboard. This is why Loc now sells the STIFFY tubes for couplers, which are fantastic and makes for the best AV bay that exists for cardboard airframes, in my opinion. Rockets could fold in half at the coupler under high G or mach flights, so we reinforced the interior of the coupler with fg and west.
To clarify, coupler failures were not "common" occurrences. Though when there was a failure on higher powered flights, it was usually due to failure of the fin/airframe intersection root or the coupler folding in half.