I compare ANTI-VIRUS programs to human flu shots. The flu shot vaccine is only good for the KNOWN viruses. Theoretically, such a flu shot would be useless for a unknown virus that surfaces.
Anti-virus software is only good as long as it discovers a virus before it travels around The Net. But how is it possible to discover a virus before it happens? It can only be discovered/addressed after it has surfaced somewhere on the internet - in which case, it may be already too late for some computers. Then comes the search for a "fix."
P.S. Despite the imperfect simile, I still recommend seasonal flu shots for humans.