Usability is primarily aimed at making things easier for the end-user. This isn't always the case, however. There are tasks, events or functions that you may actually wish to make more cumbersome or difficult.

Deleting email from a Gmail account is very difficult; the commands are buried in menus involving many steps. While we may feel that deleting irrelevant email is something we actively want to do, we might conclude it is less usable than it should be. So why have built-in difficulty and obstruction to usability? Because Google uses your body of email to mine for information it uses to target the ads it delivers to generate revenue; indeed, deleting it would be detrimental to the service. By taking a wider perspective, Google is asking us to question our assumptions about the way we deal with email.

The above is from an article called "Slanty Design" from the January issue of Communications of the ACM. The full text can be found here in either PDF or HTML.