Why do people insist on server sessions?

Some years ago I was asked to help to create a functional tests infrastructure on an web application project. I've started doing the preliminary tests using badboy, and soon aborted my mission. Why? All of the context between pages and flow was defined over server session, creating an unnecessary dependency over the session, hiding valuable data from the tests and creating dangerous side-effects and dependencies between parallel client sessions.

We are starting to discuss our next test automation tool - which will probably be Selenium. Our next discussion may be: will the interfaces need to be architected according to test automation requirements/limitations? And the answer will probably be positive.

[update]
Now that I think of it, the session was not the reason for failure - on some cases session can even help on these kind of platforms. Nevertheless, using session like there's no tomorrow is just a bad idea. We should define our interfaces in a clear way and isolated way, and sessions just mix everything on the same pool.