I understand speed - I use vim for quick edits/changes - but simply being able to jump to the source of a function you're looking at is huge (yes I know about ctags for vim - doesn't even feel like it comes close).
EDIT: One of the other huge benefits of IDEs in general is working with and studying code that you didn't write yourself. All the sublime and textmate fans I know are almost always just writing their own code from scratch, or using a small subset of well-known plugins for popular framework X. Taking code written by someone you didn't know, who perhaps didn't even know language foo very well, is a pain that is lessened somewhat by using a capable IDE vs a plain text editor.