To make it easier, I think we shall separate Defense+ from antivirus. Looking just at antivirus, you might want to use BOClean as it might have malware signatures your antivirus (whichever it is) lacks. It's true though that Defense+ adds a lot of security. Difficult question you have, a simple answer is: the need of reactive protection like BOClean and antivirus is pretty much a matter of who you are and how you use your computer. Personally I've only used CFP (w. D+) since it was released last year, and I haven't even come close to getting infected by anything. Which probably, for the most part, is a result of habits (I'm careful), common sense, and Firefox browsing with NoScript.
Hope that helped anything.
LA
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