If I understand correctly, XBAP's main pro which I believe was not emphasized strongly enough would essentially be no-deployment requirement.  Just give out a URL and you basically get a full WPF/.NET app *plus* sandboxing.

I'm not sure about installation issues though, does loading an .xbap install stuff on your machine?  (If so, how did they get Firefox to cooperate since FF will run XBAPs).

15:10 was not a particularly good example because the capabilities described there are really easily accomplished in plain old HTML/CSS.  Furthermore, if the article copy were not delivered as pure text (e.g. if it were in BAML), it would be a big mistake to go this route since the content would be unspiderable.