Definately YES.
One would be able to show one's artwork to others without them having to buy Xara just for seeing their work. Zooming in is the main issue that is of concern to me here.
I won't see a .xar viewer as a competitor to Acrobat or so. It's a bit too soon to think that.
It's more that I feel that many people (those who don't have Xara) have been left out and can't be reached to adequately show Xara's capabilities.

From a marketing POV, I think it would be a very wise idea if Xara would offer such a viewer for free so people can see what Xara users can do. This would convince them more to buy Xara products.

From my own POV, I've always wanted to show others my Xara drawings and zoom in to show more detail, but I rarely have Xara with me so I have to do with .png instead.
E.g. currently I'm working on drawing the mechanism of a certain disease which I wanna show to my colleagues for evaluation, but now I have to use flash instead because it's vector and you can zoom into .swf files (.png is not much of an option). But that's not the perfect solution either.
I'd like (but that's just my opinion) the .xar viewer to be a single .exe which you can carry around everywhere.