Editing HTML directly within your ESP editor?

We use Responsys as our ESP and I have been told by the agency we work with to NEVER edit custom HTML directly inside the Responsys campaign workbook. To only edit your HTML in an external editor like Dreamweaver and then re upload your HTML into the manage content area in Responsys.

I am trying to establish best practices and wondering everyone's thoughts on this topic. I have tried editing directly in Responsys and sometimes it does not save my change.

I was wondering all your thoughts on if you do edit HTML in your ESP or not? And why?