"Most IDEs I've used are stuck in a single code window at a time."
So which IDEs have you used then? Because either you haven't used any, or you haven't even bothered to learn the most basic features. One of the most popular and well known IDE's - Eclipse - can display as many code windows as you like (hint:click on tab, Window menu, then new Editor - there, that was easy, wasn't it? If that's too difficult, you can drag the tab sideways and it'll open another editor), hell you can even drag them out of the main UI and spread them all over the desktop if you like.
"Very few have anything near to the customization you get with Emacs or Vim, you're stuck with just a few tweaks to indentation style for example."
Where do you come up with this rubbish?! I've read a few of your comments now, and either you're a troll, or totally ignorant about most of the subjects you're gifting us with your opinions about (your post claiming that Windows has no scripting language is just... wow). Honestly, have you actually used an IDE in the last 10 years? You're being beyond ridiculous.
"GUIs are from the 70s, so why not claim that those are ancient relics?"
I would if they still looked and functioned almost exactly the same as the ones we have today. The fact is, that vi and EMACs were designed to be used on dumb character terminals connected over slow serial/parallel cables to relatively slow servers. They still work well over ssh, as they're pretty lightweight and there's no need for a GUI. However for developing on a desktop, it's ludicrous to use them in preference to an IDE for anything except the most basic editing.
"...why does the AVR debugger have to be a part of the IDE instead of a standalone tool..."
Here's a clue; try looking up the "I" in "IDE".
You've obviously completely missed the point of IDEs, and seem unable to accept change, even when the vast majority of professional developers do understand it. There are many things beyond a "class browser", so break out your copy of lynx or gopher and see if you can find a "search engine" like Google. You can then type stuff into it and it'll give you information. You're different and special, and quite frankly I don't have the patience.
And I'm not a "vi hater". Like I say, I've used it for over 30 years, along with EMACs (though nowhere near as often) on various flavours of UNIX and Linux - probably a damn sight more than most people here. I'm fully aware of their limitations, and the fact they cannot compare to an IDE for serious development work. If you're using them, and still manually creating makefiles in the 21st century, you're some kind of masochist, or you enjoy fiddling around with scripts more than getting stuff done(tm).