Anti-virus is still extremely useful. It is not an end in and of itself, it isn't a panacea that will keep you safe from everything, but it is a useful layer of security. The only true defense that has any chance is defense in depth, layers of security. So that when one layer fails, and they WILL fail there's no perfect security, other layers stop the problem.
AV is a useful layer. It screens for known threats and good AV gets that list updated multiple times per day. So it can flat out stop any known threat from getting on a system. It can scan things as they download, before they execute, and block known threats.
That is useful, particularly against the kind of threats normal users face. They don't usually face highly specialized and targeted threats, they face something that sneaks in through a bad ad in a compromised ad network or the like.
We make plenty of use of AV at work and it has done a great job cutting down on compromised systems, and cleaning up systems that do get compromised (which generally don't have AV). I certainly wouldn't rely on it as the be-all, end-all, but it is a good layer of security.
It's also a pretty cheap one. You can have MSE for free, which has about a 90% catch rate, or for $40ish per year you can get one with a much higher catch rate (NOD32 being my preference). That's not a bad price for a useful layer of security.