Yahoo announced they are reducing their user data retention policy to 90 days. Read about it here. Is this good or bad for users? Theoretically the more information a company has about you the better it can service you. The company knows your interests and behaviours and can therefore optimize the relevancy of the information delivered to you. If you restrict the information to 90 days you have lost some of the history and potentially reduced the user experience.
I am hooked on Amazon for buying books. They have lots of my purchase history and book ratings therefore when they make book suggestions they are pretty close to what I am interested in. I have 'discovered' may great books through Amazon suggestions. I want Amazon to know what I like and what I purchased.
The trade off of course is privacy and this is why Yahoo is changing its policy. Perhaps the best mechanism would be to let the user decide on the retention policy therefore those who want a better experience can have it and those who have privacy concerns can have that addressed as well.