All very true. But...there's always a but...every classic vehicle I've seen on road/on track after a knock and the ARB/GRP material's fooked and it looks bloomin' awful. Sometimes it can be GRP repaired in-situ but it's very messy and takes an age to repair properly.
On my MGBV8 Sebring the front dam spoiler and rear Sebring lower valence took a huge amount of work to fit properly. I spent days reinforcing and prep'ing these panels before final pre-fit and final spraying. The rear is near impossible to see any profile join and this took nearly a week to feather-in and to ensure the join would never crack. All this from someone who knows their way around a car body/bodywork repairs with dollies, surf boards, correct build/prep' materials and home spraying facilities.
Unless full on motorsport, my best advisory for Defender wings would be buy metal.