I run Windows XP without an anti-virus and every few years, I run one
of those free online anti-virus scans. My system always is clean.
It's your PC and your choice but to me that's like saying "I haven't worn a seatbelt in years and haven't been killed in a car crash yet, so I don't see the point of seatbelts."
Anti-virus is a safety net for when things go wrong, not something that's likely to save you on a daily basis.
I have the latest version of Firefox installed
Firefox gets patched reasonably quickly but there are still exploits that exist for several days. If you have Flash, Adobe Reader, etc. installed at all then make that weeks, not days.
plus Spybot Search and Destroy's immunizations installed
Isn't that a form of anti-virus, or am I confused about what the tool does? Never used it myself.
I also run NoScript and AdBlock Plus, which minimize the surface area of the main attack vectors that such exploits use.
I don't know how anyone can use NoScript without whitelisting so many sites that the purpose of it is defeated. I tried it I & thought it should be renamed to "NoWeb" as it broke most of the Internet (especially tech news websites).
It's a nice idea, since javascript is involved in so many exploits these days, but unfortunately too many sites use javascript (not always but usually gratuitously, IMO).
Still, I agree that you are unlikely to get a virus -- as are most people who are sensible -- and it's also true that anti-virus tools aren't 100% effective and won't catch every new threat in time. But, IMO, the good a/v tools are worth the performance hit they impose.