Well, I personally believe that for our server based AV products, anti-spam would be a great added value.
I was not sure about the consumer products, though. The truth is that most people already have some sort of antispam solution in place, and also that an extra module would increase the size of avast a bit... However, if it could be completely deactivated.... why not?
Also, while I absolutely agree with the "multiple layers of defense" paradigm I've got to add that antispam is not really like AntiVirus or AntiTrojan etc. - simply because the objects it filters are passive (meaning that the spam does not actively try to tamper with the protection by trying to kill some processes etc. - i.e. in a way that would in any way influence the working of the rest of the security package (e.g. antivirus)).
Vlk