IMHO, what's more likely to happen is: You search for a camp stove, and your G+ friend has written a _public_ review of one, and your personalized search results show your friend's review as well. Personally, I would find that quite helpful since I can now contact my friend directly about this, but even if you don't, why not disable personalized results for yourself?
(To your prediction of how this feature might play out in reality I would say that you are probably right, but also that I don't give two shits about what my friends buy OR about what they publicly review.)
What we know for certain is that Google is aggressively pushing its users onto G+ and that is the real heart of the problem. It's a bait-and-switch. I have no intention of using Google as a social network and I have no guarantees that I will be able to opt out of current or future features. The writing is on the wall -- Google will be social. That upsets some people.