If you are a Rails developer you know all the conventions and they help you at finding code with only an editor and a file manager. No need of IDEs. It's very easy to learn a new application, unless the original developer wanted to be clever, which usually means that was one of his/her first Rails projects.

However there are other school of thoughts and not everybody likes the Rails way. It's fair and it's a big world with space for every opinion and tool.