Well, the first betas did _require_ it. But that of course meant problems as that created a dependency we really did not like. Other problem is that to our understanding the user should not be able to suppress at least the most important popups (malware detection) as that's why he installs the avast! to be protected, and that is impossible with Growl (or least was when we used it, not sure about current versions). So this has been removed many months ago.
Mity