hellsbrink,
Like having the commercial incentive to provide a good product, there is no such incentive if you're just pushing out free stuff.
Having a commercial product means you have to invest time in R&D, QA,providing support and responding quickly to new threats. Otherwise the ruthlessly competitive environment of the commercial world will eat you for breakfast.
This fact is supported by the independent comparatives, the top three products of 2010 were all commercial, with the top two scoring the highest ADV+ rating across all 7 categories (AVIRA and F-Secure, with F-Secure being eventually named product of the year), closely followed by Symantec (NOT the same thing as Norton !) came a close third, with ADV+ in 6 and ADV in 1 test.
Avast only scored 3 ADV+ and 3 ADV, which, whilst respectable, is nowhere near the quality of its commercial rivals, putting it in 5th place. Plus you don't get any of the commercial backing mentioned above.
I think that says it all.