I can see how it can be convenient, to be able to order books, CD's, online music, etc. from the core of your desktop. However, what I don't want, is Canonical to depend on you, the users, to buy DRM'ed music, and/or proprietary software.
I have been somewhat worried about the software center offering proprietary software, however, it seems convenient in the cases, where there is no free alternative, or when the free alternative is considerably less convenient than the proprietary version. However, the user should be properly warned, that this is proprietary software, that doesn't respect his or her freedom.
However, the idea that Canonical gets a dime or whatever, when someone uses Ubuntu to buy a new book, doesn't worry me. I wish they would extend their support on free software instead. I am sure people concerned about free software, would be willing to pay for support. We also need Canonical to offer governments, schools and stuff like that a proper alternative to the proprietary stuff, they use today.
If the idea with this thing, however, is merely to go for the money of random GNU/Linux users, who stumble on Ubuntu by coincidence, to buy proprietary software, DRM'ed music, and stuff like that, then it may very well be a cancer in the core GUI of the default Ubuntu install.