A consensus seems to be forming.
I always associate zip-off trousers with people who say things like (in your best John Major/wally voice) 'Always pays to be prepared for any eventuality!'
Assuming that it's just a slightly altered comfort-related sense of emergency.
But today, whilst out, I suddenly saw a purpose for them. I'd been walking through some thick bush, where you'd want something covering your legs, but when I got to where I was aiming for, which was open downland, I wanted to feel the wind and sun on my legs.
Anyway, I've sobered up now.
I gave myself a stern talking to, and a necessary slap, and like any sane person I can see they are the work of Satan.