It's not about the reward. It's more about making sure the system that's the foundation of all tech on top of it is robust and that no angry/jealous troll can come in and break it.
That's why we have POW and POS. POW is great, but it's too wasteful and not sustainable. POS is also great because it makes sure that the people making sure our system works has an incentive to do so because they have a large stake in the system.
With NEM's POS derivative called POI, it takes that one step further. It adds in the extra layer of not only having a vested amount in the system, but you also have to be an active user of the system by having a good amount of transactions accredited to your name. By doing this, you can just have some rich guy who comes in and lays down a ton of money and he just collects the fees into perpetuity. The staker actually has to be actively using the network as well and I think this extra layer makes it very interesting as it promotes healthy activity.