Just because your computer is running fine, does not mean there's nothing wrong. Sure it's possible you may never encounter such an issue, but why risk it? I fix computers for a living, and last month I had to spend two weeks helping somebody sort out a load of issues they had with their credit card details being used fraudulently. They had a rootkit installed on the computer that was connecting to a server in Russia and basically sending almost everything she typed, so that must have been how the criminals got her details.
For all those smug Mac users that say only PCs get viruses... sorry, I've had to remove viruses off of a Mac too! Thankfully it's far more rare, but Mac users are often guilty of forwarding virus ridden messages, and with appropriate anti-virus installed, the infected messages would be filtered out - even when not directly affected, they are often carriers.
In light of this, I maintain as I always have done, that using a computer online without appropriate antivirus and firewall protection, is totally irresponsible.