I wear multifocals, with quite large lenses, not fashionable, and they work very well for distance, viewing the instruments, and for looking at maps. Before I got the m'focals I tried bifocals but the transition line at the top of the reading zone seemed to always be in the way of where I wanted to look, drove me mad. A couple of years ago at home the lens in the right side of my glasses (my better eye) just popped out without warning. The screws holding it in had got loose. I thought, thank god that didn't happen on short final. I now always have a spare pair close at hand in the cockpit. So I can see the rationale behind having two pairs. it's not an issue $$-wise because I get a new pair every couple of years on my health insurance so I have quite a few old pairs lying around.