In reply to Peter Knight.

No it’s not. No professional developer codes on the live site. If someone does, they’re internalizing bad practices and WP should not encourage those.

@RickGregory I think you're missing the point because all you can think of this is in terms of 'cowboy coding', which is not what I'm advocating at all. The reason not to 'code on a live site' is that it directly affects the functioning of the site with mistakes showing up instantly to all users. What I'm suggesting is that down the road having the ability to safely write some theme code might be completely feasible and worthwhile. This is not something that is around the corner but who knows what happens down the line.

The customizer architecture is doing a lot of legwork already for operations that normally would have to take place on a dev or staging site. With changesets all types of WordPress users, from devs to novices will be able to stage changes to their site without affecting the front-end. That's moving WordPress to a better place.

Having the ability to write some code without necessarily affecting the front-end directly could be a logical step to happen at a later stage. I personally see a lot of value in that, because it's generally a good thing to make things that are hard to do today easier tomorrow. And it's hard/burdensome to have to create an entire staging or dev site just to write a few lines of code. And if an admin is already working from a staging or dev site, an in-app editor might be very attractive because it lowers the barrier to entry. If a host provides a staging site easily, they can write some code without having to setup tooling on their machine.

ps. In my experience the group of users most likely to code on a live site are pro devs, those that state otherwise are likely lying :)