> If we know something of your interests (based on G+ and/or search), isn't suggesting more relevant content to you on Youtube (as a purely hypothetical example) a good thing?
No absolutely not, a search that is more relevant occasionally is only slightly useful maybe getting a slightly better link a few percentage points of the time(past a certain amount how much more relevant can you get. If you go to far in trying to guess the persons mind based on persons external history rather then their search it seem like you would get more way of answers and if you don't the answer would likely have been near the top anyway), keeping less information on users for shorter amounts of time should be preferred.
No absolutely not, a search that is more relevant occasionally is only slightly useful maybe getting a slightly better link a few percentage points of the time(past a certain amount how much more relevant can you get. If you go to far in trying to guess the persons mind based on persons external history rather then their search it seem like you would get more way of answers and if you don't the answer would likely have been near the top anyway), keeping less information on users for shorter amounts of time should be preferred.