> When you need to signup for a new service on the go, you can add your backup key when you get to it
Requires you to remember doing that each and every time. Incidentally this isn't that different from just grabbing your keys like the parent suggested. Only it introduces a new variable: time delay. A lot can happen in that time and we all know the reality is that even a diligent person is going to slip now and then. It surely isn't a reasonable expectation for an average person.
every few months I swap 2 and 3, and re-enroll any missing (kept track of with a spreadsheet)
quite annoying, offline enrollment would be considerably better
It's absolutely a goal, since a PIN doesn't prevent your security key from loss, theft, or physical destruction.