Even if I published a public key, I wouldn't keep the private key on every device, and if I have to dig out my yubikey just to find out you're in town for lunch next Tuesday, we're probably not having lunch.
I never felt comfortable having my private key on my phone, so I can't read encrypted email on the go.
I get one or two encrypted emails on a good year, and they are rarely important (or require encryption really).
Well if only "important" things are encrypted, then the existence of an encrypted message is evidence to whomever you may be trying to hide from that something "important" is going on. Ideally everything would be encrypted, important or not; having all current encrypted things being unimportant is actually better than having only important things encrypted. :-)
I should have used "urgency" instead.