There are actually a couple of good reasons for an AC or at least a dual-band N router. First, if you have multiple devices on wireless, be aware that, unlike wired networking, that transmission bandwidth is generally shared among all of your devices. If you're in an area with many Wifi networks, that transmission bandwidth is shared among all devices on a common channel that can see each other. You can see why it tends to slow down quite quickly. Second, AC and N routers also tend to have better range. And last, if you're in a Wifi-congested area, there are far fewer devices and far more channels in the 5 GHz bands, allowing devices which can access that area a better experience overall.