When "anonymization" is done poorly, it has consequences.

It was bad enough with Netflix recommendations getting de-anonymized; imagine what could happen with actual health records leaked.

http://arstechnica.com/tech-policy/2009/09/your-secrets-live...

The anonymization rules for PHI are strict enough that it would make a lot of the interesting mining you could do difficult if not impossible. Specifically the restrictions related to dates and locations.

It's a lot trickier than this. Suppose you have a rare genetic condition that affects 0.005% of the population. It takes very little additional information to single out a person when the first thing you do is rule out 99.995% of the possibilities.

According to who?