Having it negates the "not needing it" selling point IMO. In addition, it being worthless and taking up space and CPU cycles (however passive it is) is a negative in my book. I don't see how it's a selling point at all.
Most users will see any security feature as a net positive. Note the positive publicity (amongst normal users) for Leopard ASLR, a feature that adds no real security. Adding basic anti-malware features to the OS will not negate any of the security value prop in Snow Leopard.
I share your take on antivirus software, but do note that preventing a user from installing something with a known virus on it does not have negative utility.