I use Emacs and sometimes I open a window to bring up a man page or code definition for some function, but I've never felt the need for a fancy IDE. You just get used to what is going on in the code. Like if you are a fiction writer and you're asked to write a new Star Trek episode, after watching a few existing episodes you would get familiar enough with the characters and milieu to not need to refer to the series bible too much. An IDE that pops up suggestions like "Live long and prosper" or "we have 93.2357% chance of blowing up the ship, Captain" when you type the name "Spock" sounds counterproductive. Maybe I'm missing something though.

Added: it helps a lot to have real documentation, something that has gone out of fashion in recent years. Maybe these IDE's are being used as a substitute. I hadn't thought of it that way before. Hmm.

Added 2: Also, Emacs has always had TAGS (this quick navigates the editor to a function's definition given its name) and I do use that sometimes.