First, I use XWD for much of my website design work. The issue is that the mobile phone and computing market is exploding. Designing a single site for both large and small screens is basically a css and user agent detection problem but takes lots of time to create and is doubly difficult in XWD because of the inline css that it produces. Providing a workflow in XWD to design for small screen devices would be a terrific feature that would save lots of time.