Aside from helping the network, there is no incentive to run a full Bitcoin node. But it doesn't take much in the way of resources: a few tens of gigabytes of disk space and a few tens of kilobits/sec of bandwidth. (Don't forget to open a port on your router!) I do it for the same reason I give over an extra SSID to openwireless.org, or run a TOR exit node, or seed any file I got from BitTorrent for a while afterwards: it helps the network.