Although the editor inside Responsys campaign workbook is adequate for minor changes and/or additions (like for example adding a personalization token), it is not very useful when it comes to editing complex/large email content.
It lacks features like proper html formatting, searching, advanced tag completion, section expanding/collapsing, visual aids (e.g. color coding), code versioning, etc.
I would advise you to build a development routine where you do all the "heavy" stuff in an external HTML editor and then transfer your HTML code in the built-in HTML editor for further additions/testing. It all comes down to personal preference but you will discover that a hybrid approach yields better results.