I don't know if it's that straigh-forward as "not core to their revenue stream". At the time it launched I guess it was supposed to be part of their core revenue stream -- it was supposed to give them more insight into what their users are into, and thereby allowing them to target their ads better. Not too different to Google+, I guess, just a different wrapper. I think the problem here was that it didn't catch on as well as they wanted it, and even people I know who were using it, moved on after a while.

So, I guess it would be more correct to say that you shouldn't use Google products that aren't going to be popular enough. Unfortunately I don't have such an insight into the future.