Firefox was started when Mozilla (Communicator) grew too large. Let's put everything in plugins they said! Do even 5% of users need dev tools? History repeating itself.
It could be argued that integrating basic developer tools is also a statement about the kind of internet we ought to have: One where people are as empowered to understand and contribute as they are to consume.
I've seen not-terribly technical people— or at least people who'd never otherwise have firebug installed— go and edit braindamaged forms to get a webpage working right. Having it built in is practically empowering.
Also, bundled developer tools can be nice when you're trying to debug an issue on a user's machine. It's a small thing, but it's nice to not make them install some plugin.
At this point, developer tools are essentially a required feature for a browser. All else aside, there's no way to debug a bug that's specific to a single browser without tools integrated into that browser.
