I've been told by IDE users that a large part of why they use IDEs is just how much work it can do for you without you having to think about it. Need to change a class name that gets referenced hundreds of places throughout a project? Project-wide search and replace! Can't figure out what the string of 4 integer parameters to a function are? Just hover over it with a mouse and the IDE will tell you! Some IDEs even go so far as to create code for you using templates. And then there's always interactive debugging.

But, all this wonder comes with a price. For me, the key reasons I prefer vim to any ide out there is:

1) speed. compared to vim + bash, every ide is slow. 2) code portability. I work with lots of different languages on a daily basis and having to switch from Eclipse (java) to Monodevelop (C#) to whatever the http/css/php du jour is these days. 3) IDE maintenance. No matter what you use, ides have a ton of setup overhead just for the specifics of the project you're working on, let alone if you want to customize the environment. 4) fragility. Vim + bash rarely breaks or leaves me unable to work on a project. I have been left high and dry by IDEs multiple times. All it takes is a coworker checking in personal changes to a team-wide project file. Then you're left in a broken state, figuring out what went wrong.

so, there are some pros and cons. Everybody's style is different and there are real advantages to both approaches.