Re: FAO Glasses Wearers
Too much eye relief can make it very difficult to keep the right distance. Using an eye cup restricts air movement, and the combination of warm, moist eyeball and cold glass surface is a recipe for condensation which is best controlled by ventilation. It's a fairly serious issue.
It is much easier to deal with too long an eye relief than too short! Astronomers spend a long time looking through their 'scope and can use a properly shaped lens cup if they need to exclude external light
Another point against using spectacles at the eyepiece is that it's very easy to scratch the spectacle lenses.
As for designing for sufficient eye relief: compromise, compromise, compromise: you either make the thing usable, or you don't; a compact, lightweight viewfinder that you can't use effectively isn't a sensible compromise IMHO.
If you're not living on the edge, you're wasting space