Gmail – Why does Facebook suggest friends that only appear in the Google Contacts

First, Facebook as a whole is more valuable if it maximises user volumes. So they're never going to stop wanting you to add that new friend from some other address book.

Second, the Facebook experience is pretty lame if you have only a few Facebook friends. So creating a new link may be a marginal value gain for you, but it may help the other party get into the sweet spot and become a regular and committed user.

More speculatively, Facebook ads themselves seem (from what I get offered, anyway), pretty feeble, so maybe there's an element of Facebook improving your general perception of the right-hand column by sometimes not even showing you some horrible, mis-targeted ad and just doing self-promotion, not caring all that much whether you respond. In this case, you've responded indirectly by talking about Facebook, I'm making a mental note to see if I am getting that same treatment, and we're both even more bought into the idea that Facebook is an unassailable, valuable site in a way that Myspace and other toppled social media towers must not have been!

These contacts are apparently separate from the ones added by Google+. If I use the find duplicates feature to merge them, will their phone numbers now be associated with their Google+ profiles in Google's databases? Will this information be visible to other people? In short, will this violate my friends' privacy?

No, it depends on their privacy settings.

I seem to remember some scare where Facebook might or might not have grabbed phone numbers and associated them with Facebook profiles by picking them out of friends' phonebooks on their smartphones.)

That wasn't a scare, neither were the numbers "picked up", it was opt-in.