Not sure sure if you know about this but they reason they don’t allow it in other clients is the encrypted portion. However, they built a bridge recently that allows you to use it within other clients on Linux (not sure what other OS but looks like windows too) and I’ve been running it on the Evolution client since.

Proton Mail Bridge

Same here. Although I have no real reason with my threat level. But I could see how it could be relevant for some people. I also liked the idea because regular people could potentially use your thumb print when sleeping or something so this stops that Initial unlock.