I agree the paid software being advertised in the software center is pretty lame, but the rest? It's very easy to turn off. Takes roughly 4 seconds. For what Ubuntu offers, I'm ok with spending 4 seconds out of my entire Ubuntu life to make it just as "free" as Debian. I'm sure others will feel different (#linux sure does).
It's easy to disable, though I'm in the camp that it should be an opt-in feature and not an opt-out one. I don't use Ubuntu, but not because of the Shopping Lens. I find it slow, unstable, and a resource hog. I recently switched to OpenSUSE 12.3 and I couldn't be happier.