I said, "
Almost every feature ...", and you probably found a few it lacks. I'm not talking in absolutes as that rarely produces an interesting discussion. Thus I don't really see your point, if anything you helped to prove mine, as hundreds or thousands of other features are available out-of-the-box or on Emacs wiki, github, etc. Additionally I don't understand why one needs IDE support for something `grep -Hn` can do, but to each his own.
We might use different technologies. I mostly write Objective-C, Ruby, Python, C, JavaScript, and recently some Objective-J and Erlang. Now and then I write Lisp (CL, Emacs), Scheme, Haskell, x86 assembly, and PHP. The only one of those I don't write in Emacs is Obj-C.
Other editors or IDEs may be better at certain tasks, just as Emacs has unparalleled Lisp support with SLIME. When there is clearly a best tool in one field then people tend to use that tool, if the payoff is worth it. Except for pg and a few others, people generally write Lisp in Emacs. I write Cocoa and Cocoa Touch apps in Xcode and IB. I'm pragmatic about it. But when I tried to use Eclipse for Mojo (webOS) because Palm recommended Eclipse + Mojo plugin, it ended up being more productive for me to extend an existing Mojo mode for Emacs[1] to have even more features[2] than the Eclipse plugin (or any of the others, Komodo, etc). Similarly if I were doing Java I would at least look into IntelliJ IDEA or Eclipse.
[1] http://www.emacswiki.org/emacs/MojoSdk and http://github.com/samsonjs/mojo.el
[2] http://www.webos-internals.org/wiki/Comparison_of_Editors (temporarily down)