This isn't quite "hard evidence", but as an...experiment...a few weeks ago, I added every single person Google recommended, a total of 1118. Of those, 65 complete strangers have added me back, which is 5.8%. This percentage is not "most", "many", or even "some". It is irksome to see the red notification for no good reason, but I do not think people really feel compelled to add random people to their circles.
Not long at all, actually. The first time I added people, Google was recommending around 600 users. So I just selected all, the dropped them into a new circle. Same process to get the rest of the remainders over the next few days.
Now, Google shows maybe 100 or so recommended users, which is below my threshold of "fun number of people to add at one time". I say "maybe 100" because, interestingly, Google+ no longer shows the number of recommended users. I wonder if they removed this feature because of visual clutter, or were finding a lot of people abusing the feature like I did.
