What’s the risk?
You may wonder what the fuss is about anyway. So what if Google scans your photos to find out that you like to wear Nike trainers, and then uses that information to target a Nike ad at you? Photos actually has the potential to enhance Google Now considerably. The more Google learns about you, the more accurate and helpful its predictions or suggestions will be. Where’s the harm?
The risks here reach beyond a simple point of principle about personal privacy, though that will be reason enough for some people to avoid the service. Information can leak, or be stolen. Companies can share data behind our backs. Government agencies can snoop. Google can decide to leverage legal rights you’ve effectively already given it at any time. That doesn’t mean to say that it will ever happen, but you can’t rule it out.
Realistically, it comes down to trust. Do you trust Google?