It's yet another hoop to jump through, one that further splits "developers" and "users" and makes it harder to be a "casual developer" - one who just wants to make an extension and share it among a small group.
That's a fair point, but I don't think it's that bad. I do this all the time with Chrome, which already has walled garden:
"Hey guys, I made a Chrome Extension that inlines all the images in our shitty issue tracking app rather than having to download them all. Extract the zip, visit chrome://extensions, enable developer mode, and load that folder."
For a Jira-fixer extension, we'd be running FF Developer, but yes, if I did want to distribute it wider, then signing would be reasonable.
Either you are a bonded commercial developer, or you are a user (or maybe sheep-er?).