AirResearcher has raised some good points there.
The samples community is quite tight knit, and the commercial sector have a greater number of avenues available to them for accessing samples than the freeware community.
However it is, of course, important to remember that the more serious virus writers have access to the anti-virus software and devote much time to trying to make themselves invisible. Hence having the commercial R&D muscle behind you to continue to innovate new detection techniques is important.
Furthermore, if you are selling AV software, then there is a commercial incentive to produce quality software, otherwise market competition will dictate your fate. If you're giving away free AV software, there is no such incentive, you're not getting revenue from your software anyway, so what are you going to miss if people stop using it ?