Refactoring. In Visual Studio I can click on any usage of a badly named class, press F2, type in the new name and it will be updated everywhere. Even the name of the file holding the class is updated.
More importantly, it won't be updated for things that aren't it. As in, you can have a class called 'i', used in 50 files, and confidently replace those usages and only those usages with another name. That's something you just plain can't pull off with Regular Expressions.
[waits for somebody to post the 4000 character Regex that actually does this correctly]
