I can't really see Google proper abusing this, as the product is one-to-one the monitoring: commerce. If you accept that, Google Now is just really targeted advertising—a big, NLP-based, bayesian coupon.

It seems like the bigger issue is when malicious parties usurp this data and use it for their own ends. We can look to science-fiction for predictions. Scroogled comes to mind, as well as a (possibly falsely remembered) recent campaign IRL to root out dissidents via internet searches.

And one more scenario: false flag. Since search records are considered evidence, planting this evidence could be all the sway that a jury needs, or even just for defamation purposes.

I think the fear is not of Google, but of making our lives tied to a record-keeper that could potentially have no accountability.

What happens when you perform a one-off search? For example, lets say a friend asks if you know the score to the Yankees game? You really have no interest in baseball, but you do the search anyhow. Should that really influence your Google Now profile?