One of the potential issues is that companies tend to leak information. To your point, while you may not want your old fellow high school students finding you, a quick bit of data mining on your profile could probably guess fairly quickly and present your profile to those individuals without your consent. (Obviously, mileage will vary depending on how much information you've elected to share, but it's fairly disturbing how easy it can be to isolate individuals in a fairly large group off of only a few small traits.) Just look at how the Netflix Challenge data was matched to individuals based off of very meager information.
The bigger issue at hand is that when you start adding up all the various public bits of information, things can get rather scary. Did you know that in Canada, a credit card company could determine potential credit risk based off whether or not an individual bought a drink in a particular bar in a particular city with fairly impressive accuracy? http://consumerist.com/2009/05/your-credit-card-company-is-b... I could easily imagine that an insurance company may want to determine risk profile for customers based off of browsing history and age, or possibly an online retailer may alter prices slightly because they know you have more income to spend and a history of not checking prices that aggressively.
The larger a presence you have, the easier it is to do that sort of thing.