Should developers touch XAML?
However if you read the above blog by Scott, it has merit and definitely there are issues with respect to appropriate working together of designers and developers. However there is another angle to it and that is more evident if you have attended (or seen the recorded session) of the talk Seema had given in PDC 2008. You can find the link to it in my this blog.
Tools like expression when used to generate XAML may not generate the most efficient XAML, especially as you try to record some animations, or create grid columns by dragging column separator or create animations etc. The various values that get written in XAML can definitely be subject to more optimization to help gain a little more runtime mileage.
And hence develper may also need to edit the XAML and in this case, it may literally be editing the XAML in XML view in VS/Expression and not in designer view. However this can lead to subtle issues as well, which you should be careful about. See a similar connect feedback item I had logged for Expression generated animation here.
Personally, I don't completely buy into the MS comment on the connect issue. It would be interesting to hear your thoughts on the same.