As stated above, no technical reason, but…

Anti-malware technology MUST evolve to handle the latest threats. Taken to an extreme (staying with a “comfortable” version forever), it would cause you to fall so far out of date that your anti-malware software might completely miss newly crafted threats. And thus you could be infected with a virus or worm or worse and the whole reason for having Avast and it’s auto-update facilities would be lost.

Having the Avast team keep on top of threats so we don’t have to is a very powerful thing.

Just something to keep in mind.

-Noel