Your question opens up a can of worms. Ideally any operating system should not have any security flaws, but then this kind of OS will probably be available only in Heaven. In reality most OS's aren't perfect and there are people out there whose only job in life is to find these flaws. now, some of them are motivated by good intentions, they want to identify the flaws and let the developers know so that they can plug them. others just want to exploit them.
The problem comes because however you look at it, there will always be a delay between a security hole and the update plug for this hole. Ideally this update will come within a few hours of the hole being identified, but then we do not live in an ideal world.
What this means is that by the time an update comes, you have already been running the system, with the hole for anywhere between a few days to a few months or more. So one more day is not going to make much of a difference, is it?
Is this ideal? Definitely not. But that's what we get and that's what we have to live with.
And just so that you don't get the wrong impression, just check around as to how many security flaws Windows has plugged and how long they have taken to plug this. You'd be surprised at the answer. People in fact don't like to use ie anymore because some issues have not been taken care of across a few upgrades!
And Mac is no different either. If check through articles in ars technica and OS news you'll find how many millions of systems have been infected. And to make matters worse Apple actually advised its dealers not to do anything about the infection even months after it had become apparent that there was a virus because they did not have a plug for it.