Do only the naive pay for anti-virus software?
The word "naive" suggests a simple, child-like mind that lacks experience for good judgment. There are probably millions of people who think the same of people who pay to have their car's oil changed. I would call the user who uses no anti-virus (or never changes the oil) is the naive one.
By far, most of the big name pay-type anti-malware solutions are excellent at what they do, and provide no problems for the vast majority of their users. And you noted yourself that you have not had any problems. Users who are "security aware" and comfortable with their security defenses are (correctly) concerned about changing to something new. Either they are (rightly) afraid of getting infected, or they heard all the (real) stories about really bad problems people have had trying to switch.
Isn't that why you ask the question? Note there are many, if not most computer users, who simply see their computer as another appliance in the house, an appliance that should just work.
There have been problems with every anti-malware solution - paid and free. None are (nor should ever be assumed) perfect. I dumped McAfee, Norton, ZoneAlarm, AVG, Comodo, Avira, and few more in between over the years because of bloat, hogging resources, or other problems. But for over 2 years I have been using MSE, Windows Firewall and IE9 on all my Windows 7 64-bit systems with no problems. I occasionally run a supplemental scan with MBAM for a double check - but it has never found anything. I also have adblockIE and I use MailWasher Pro for my spamblocker/primary email handler.
On the other hand I avoid sites that could cause problems: porn, pirate, foreign forums, etc.
And that's great, I hope you don't participate in filesharing via torrents or P2P sites either. Are you the only user of that computer? If not, are all users that disciplined?