They do ship product for Xara Xtreme because there is a lot of clipart, fonts and other extras including tutorial disks and a printed manual. IIRC the downloadable version is a stopgap while you wait for delivery but it doesn't work forever!
Also note that from version 6 Xara have introduced an activation system which I have found a bit flaky to date (I have had to activate one product more than once on my system because it lost activation). They also have a written policy that they only allow 3 activations for a product (irrespective of the reason) so if you are someone who reinstalls your system regularly this could have an impact. I haven't had to test their activation policy but I did send them a strong letter of complaint. I would object less if there was a way to deactivate the program and restore you activation count.