Why EJB?

I have been working in the J2EE environment my company selected. And we use EJB etc. for quite a while. Today, I got into this discussion with my friend, and she is suggesting EJB isn't a must in J2EE applications, and in fact, for those application logics, it is just fine implementing them in Servlets/Web Apps rather than in EJBs. EJB has all the overhands that servlets can avoid. I am confused. Thus started my quest for the purpose of EJBs.

Here are several links that I found relavent:

http://www.javaworld.com/javaworld/jw-12-2001/jw-1207-yesnoejb.html
http://www.jguru.com/faq/view.jsp?EID=126400
http://www.javaworld.com/javaworld/jw-11-2004/jw-1115-ejbtest.html
http://wiki.cs.uiuc.edu/cs427/Spring+Framework+Introduction
http://www.theserverside.com/articles/article.tss?l=SpringFramework

And obviously, someone wrote a book about it
Expert One-on-One J2EE Development Without EJB, Rod Johnson, Juergen Hoeller (Wrox, June 2004; ISBN: 0764558315)

Seems like I need to understand the Transaction and Security Supports for the EJB, and familiar myself with the clustered deployment scenario.