Personally, I absolutely love that developer tools are included in all browsers. I think its absolutely fantastic, just surprising to me it ended up that way.
The other vendors were just playing catch-up, especially Microsoft.
Plus it was handy for their own devs and aligned with their strategy (make the web awesome because more web is more Google ads).
Only one of the browsers I regularly use contains dev tools (Firefox); there are no dev tools built in to w3m, dillo, conkeror or lynx.
There were also no dev tools in eww, w3, konqueror, arora, midori, links, elinks or netsurf last time I used them.
In fact, I've only ever seen dev tools in two browsers, Firefox and Chromium, although my observations are biased since I only use Free Software ;)
The parent just-so-happened to make a tangential comment about "all browsers" bundling dev tools, which I found quite short-sighted. If you're QA testing a commericial Web site then by all means use "all browsers" as a shorthand for "the popular browsers". But if you're commenting on the general development of browsers and their features, such statements need to be qualified. After all, there are some good sibling comments discussing WorldWideWeb, Netscape, etc. Should they be dismissed as "edge cases"?
Maybe instead of listing the software I actually use, I could have taken a guess at how many mobile browsers lack dev tools; I imagine there are many users of such browsers, which is far from an "edge case", although I avoided making such conjectures since I've never browsed the Web on a mobile phone.
Others don't have features like DOM, Javascript, etc. so there's not much they could offer that "view source" doesn't already do.
Others don't have a suitable interaction model, e.g. w3m is effectively a pager for turning HTML into ANSI escape codes; conkeror is keyboard driven so would need a radically different UI for it to be effective; etc.
Others, like Netsurf, Konqueror and the Webkit wrappers (Midori, Arora, etc.) would probably like a dev tools feature, but don't have enough developer power to implement one.
Part of the issue with a lot of these alternative web browsers is that they don't have fully-fledged JS engines, or WebGL, or a number of other features that web developers like to use to make their sites fancier (and more bloated, etc etc).