Avast alerts users for critical updates. That is all. Nothing to do with piggybacking, just for the software you have and is outdated, meaning risky.
It is a service like we know from Secunia's, but now it comes build into your av solution. I think it is great, because a lot of users will get infected because lots of people are sloppy with updating/upgrading (OS, browser and other third party software),
polonus