Well, if you look at AVG you'll find that their paid-for Internet Security suite is a lot more than just virus/malware protection. If you need those features then pay for them by all means. The other, very large, group of users who will have to pay for anti-virus software are corporate users. Most free software is for home use only.
Read a few reviews of the various offerings and make up your mind whether you need the added features of paid-for products. But the free products (as long as you choose the right one) are plenty good enough protection for many people.