Trethevy Quoit 11
Image by stonefree

... but you might think it worth the effort if you were going for structural longevity (which Stonehenge reeks of IMO)

If it was a symbolic structure - which we may assume it was - then the race wasn't for time, nor longevity, but for the limits of endurance and effort. Putting the largest possible capstone at the most impossible angle, maybe.