A basic land line in PA with no features costs about $11/month after taxes -- the same as the most basic VOIP plans. For that, I get proper 911 routing and I don't have to mess with a UPS just to call the electric company when the power goes out.
There's also a problem with bandwidth and latency using VOIP. If you have a good internet provider and you don't use your internet connection while you're making a call, it sounds fine. However, my cable provider routinely has ping times in the 1-2 second range, and the line's oversubscribed enough that dropped packets are common.
Finally, to have VOIP, you need a real, routable IP address (that is, not 10.x.x.x or 192.168.x.x). A lot of ISPs, even broadband providers, won't give you one, or they make it very difficult to figure out how to get one. I've seen people who work in tech. support give up on VOIP because they got sick of fighting with the ISP and messing with firewall settings. I wouldn't even try to push VOIP on tech-illiterate family or friends.