Emacs is not an IDE- its a text editor. It would be nuts to use Emacs to edit Java compared to Intellij or even Eclipse. Having said that, it is no more productive than any other text editor at editing text (as compared to Vim or Sublime Text 3). Even if that weren't true, would you really invest years into learning a new editor to make editing text a few percent faster? Programming is limited by thought, not typing (common pain here is solved by plugins anyway). The benefits of using emacs are as follows:
1. terminal based
2. key chords
3. (e)lisp
4. open source
5. stable
6. extensible
Last I looked Emacs comes with a million lines of Lisp code extending Emacs way beyond normal editing.
Emacs easily is a text-based IDE. I for example use the SLIME extension with GNU Emacs to make it into a Common Lisp IDE.
Now we can ask if the depth of GNU Emacs makes sense and if the UI usable. But there is little doubt that Emacs is WAY beyond being a simple text editor. See ORG mode, GNUS, ...
1. terminal based 2. key chords 3. (e)lisp 4. open source 5. stable 6. extensible