Would be great for sure!
Alas, from a technical perspective, my understanding is that the way xara generates html uses absolute coordinates and external stylesheets etc. which is incompatible with the way email clients work.
Someone from Xara could officially confirm this though.
There is definitely a market for WYSIWYG email designers. The few I've tried are fiddly to use to say the least, so if they could extend (or restrict) xaras capability to render email compatible HTML that would be great.
I guess it would require an 'email mode' that would restrict/hide functionality that would not work with email clients? Or perhaps the it would need to be built using 'email compatible' smart shapes?