I maybe misunderstood but I caught the whole meaning. About deep infections, I understand it as mostly is a matter of removal (If you mean this). About highly advanced malware, what do you mean? Do you refer to malware different from all those things come to millions of home PC's?
What I wanted to compare, was only the application and it's tools capabilities, not the whole service quality with further support etc that a vendor may or may not offer. Maybe it was a fault of mine not to make this clear.
Before I forget, its a double edged sword, as free antivirus does offer you a minimal up to average level of protection.
And even tho littlebits is totally correct I must say that those infections he is talking about are more user behavior errors.
Afteral most of these people fail to stick to common sense and online practices.
I am sure you do know Symantec Security Response right? For this example I took Trojan banker as a example:
http://www.symantec.com/security_response/writeup.jsp?docid=2011-052405-0550-99
You will see that Symantec does classify it as LOW risk.
But do you have any idea how many people did class this Trojan as a true menace?
Remember Trojan Netsky? it made world wide news as one of the most dangerous Trojans on the planet (From a user point of view.)
it was still classed as a LOW risk up to its most dangerous version medium risk.
Now when I am talking about medium / high risk malware then you will see that getting infected by those is a bit more complicated, as it does usually require weaknesses within your OS, Network, Firewall, Individual software, injection and hacking attempts to plant these viruses.
Or you just allow the file as it was hidden within a seemingly legit program.
Now where free antivirus very much has a problem with detecting, removing and undoing the damage caused by these types of malware.
But then again these malware are not just infections on their own, they are planted so there is a whole chain of events which traditional free antivirus cannot deal with.
And the average paid solution has just a bit more tools and is just a bit more sophisticated to deal with these dangers, both in detection, removal and cleaning up / undo the damage.
I know its not entirely accurate what I just said, because it would require another 5 books to explain it in detail but the general sense is spot on within the short reply.