But, why would I use an IDE if I have access to a proper text editor like vim or emacs (I use vim, but I've used emacs enough to know I could be productive in it if vim didn't exist)? Your question makes no sense to me.
I've known a (very) few people who are productive in IDEs, but generally speaking, the best developers I've known use either vim or emacs. I've never experienced an IDE that didn't make me angry, so I've never spent any significant time with one, so I don't know what the valid arguments for using them are, though I'm sure there are some.
I'm curious about the framing of your question. What is the rationale behind your question? When you use a big IDE do you feel more productive? Do you just want to avoid learning how to use vim or emacs? What are you trying to achieve here that brings up this question? Do you think an IDE will make learning to program easier?
One of the reasons I've never used an IDE heavily is because I find them very difficult to use and understand. They pile together a bunch of different concepts under one roof, and assume that it fits the work I'm doing. It pretty much never does, so I would just be using it as a big slow editor anyway. The other reason is that whenever I've wanted something IDE-like (like completion, or highlighting and indenting for various syntax, etc.), I've easily found a plugin for vim to do the same job without all the extra baggage.
The answer is generally that IDEs are big, slow, cumbersome and distract from the software you're building. If you're working in a language where the assistive tools are a core part of the experience (Java and C# seem to fall into this category), then maybe you should use those tools. I work in dynamic languages, and big slow editors just get in my way, slow me down, and fight against my own toolchain and workflow (which includes shell scripts, the shell itself, the ability to open and edit files from anywhere, including in-place on the server).