The Expectations of Links
When developing a web page, the user's expectation of what a link does is well understood. Clicking on a link will take you to another page. Within a web application, like Google Mail or Basecamp, links have much more power. They can perform changes, log you out of the application or even delete records.
On one hand, should links even have this kind of power? Links are easily acted upon by search indexers, site copiers, and the performance-enhanching Google Web Accelerator. Each one of these could inadvertently do damage to your application.
John Gruber once said, "the location field is the new command line." But take a look at the actual URL these days and you'll see how true this is. Frameworks like Ruby on Rails are creating easy-to-read and easy-to-follow URL's like "http://www.example.com/email/190/delete".
The HTTP specification recommends against using GET for this type of functionality but it's certainly convenient. A link can be small and unobtrusive. It's scalable. It's a mechanism that people understand. Does the average person understand the difference in functionality between a link and a form submit button? I suspect not, nor do I think it's important that they do. What I do think is important is that a mechanism exists that prevents accidental harm.
The issue is bigger than some automated tool randomly clicking links on you. There's also the harm done by users who've accidentally clicked on links or buttons that they shouldn't have. And in this situation, it doesn't matter whether it's a link or a form button submitting the information via POST. What matters is that a user has initiated an action that they didn't want. In Hotmail, for example, when viewing a message, the Forward button is right next to the Delete button. If I were too quick or my mouse button stuck for half a second then I might hit that Delete button. With no confirmation, my email would disappear in a flash. (In this particular case, Hotmail has a Trash Can from which I can recover that email.)
Allow me to provide another example that has happened to me on more than one occassion. I often like to use the keyboard or scroll button on the mouse for navigation but the document has to be selected before I can do that. I have to click within the page to give it focus. Every now and then, though, I'll end up clicking some random link or advertisement (yep, you've gotten a few clickthroughs from me!). This particular example is pretty harmless until that one day I accidentally click on the wrong link!
This is also an issue that affects some of those who are physically impaired. Hitting a spot on the screen can be more difficult and that person has a higher rish of accidentally hitting the wrong link.