Not only is there no reward but under the currently proposed TF there is actually *zero* benefit (i.e. they are not helping to secure the network at all).
While they are not helping to secure the network, they are helping to ensure the functionality through providing bandwith. With their bandwidth, downloading the blockchain becomes faster for other clients. When bandwidth runs low, nobody can use Nxt as you would never see your confirmed transaction in the blockchain if you cannot keep up downloading with the blockchain's growth.
Edit: Its also more secure to have a lot of nodes providing bandwidth rather than a few very fast nodes. A few nodes can be tricked into providing false blockchains easier than a loarge number of independent nodes. So more nodes equal more security.