The dev tools can be very useful to average users, even if they don't use them the same way that developers might.
For example, I know several non-technical users who use the built-in dev tools to remove the annoying overlays that some sites show, but that don't allow to be hidden without logging in or something dumb like that.
These users don't really know what the DOM is, but they do know that they can open the dev tools, click the node selection button, click on the overlay they want to get rid of, press the Delete key, and now they're able to use the site again.
Firefox's DevTools can render a whole web page as an image to disk (not just the visible viewport, like screenshot tools do). Quite a few non-developers have found that useful when I've showed this feature to them. No extensions or 3rd-party software required.
The built-in dev tools are also a social-engineering attack vector. Try opening the dev tools on facebook.com and you'll see a huge warning to users that have been told to copy/paste malicious JavaScript into their dev tools. So one could make a reasonable argument that built-in dev tools are a bad thing.
Funnily enough, Facebook is one of the sites where I've seen these non-technical users have to use the dev tools to make the site usable again!
I think it was a login prompt overlay shown on business profile pages. These users didn't want to log in to Facebook, yet needed to view the business' Facebook page. The overlay would cover a significant portion of the page, without any good way to get rid of it. So the users would use the dev tools to clean up the page, essentially, even if they didn't really know what they were doing.
For example, I know several non-technical users who use the built-in dev tools to remove the annoying overlays that some sites show, but that don't allow to be hidden without logging in or something dumb like that.
These users don't really know what the DOM is, but they do know that they can open the dev tools, click the node selection button, click on the overlay they want to get rid of, press the Delete key, and now they're able to use the site again.