Your logic is flawed, too.
That means you admitted your logic is flawed?
Anyway, risks are always there and you're absolutely right about it.
But it's all about to reduce the probability of "unnecessary" risk, and to
minimize the damage when the misfortune happened, right? Especially for
those we can't afford to lose or the costs will be too large? (e.g. data and
work hours)
Safe practice such as good driving habits, and in the case of using
computer, such as good programming practice and those you already mentioned
are active defense systems. But why do car manufacturers put air bags and
seat belts and so on? Those are passive defense systems for accidents that
are not caused by one's good driving habits. In that case, it's to reduce
the damage.
For virus infection, it's more than just how one wishes to use his/her
computer, for which case, I will totally respect one's choices and will not
join this thread. But if one gets infected, the system may become a bridge
for passing the virus to other systems, and that becomes a public safety
issue.
Admittedly, even AV is not 100% bullet proof but we can at least minimize
the risk to certain degree without exposing ourselves to all known and
unknown viruses. And if we are unfortunately infected, we could quickly fix
the problem or minimize the damage until a professional comes to rescue.
Vista has improved security and that's out of question, but it's not
invincible and I don't think any OS is invincible.
Honestly speaking, if your infection won't endanger others or if a reckless
driver won't hurt other innocent people or passengers in the car, I won't
care about it at all. But you may wish to reconsider not just for yourself
but also for others?
Unless of course, your system will never be online and you will never pass
any files from your system to others, it will be totally ok.
I think that I have made all necessary points, and the final choice is
yours, of course. So further decision may not be necessary.
Good luck.