Perhaps I am misunderstanding something, but this seems to idealistically ignore a different sort of selfishness.
If I'm running a Nxt-based business, I could either invest in forging myself, or just not bother, and expect someone else to do it.
A business that forges for itself, with minimal mining rewards, will be less efficient than one that just expects someone else to do it.
The free-riding company will out-compete the 'honest' company.
Agree with ChuckOne. Look also at the example of bittorrent: the torrent trackers had to use rating systems and such, just because otherwise there would be too many leechers for too few seeders. That's not because it costs anything (at least in free countries like Brasil, where there is no risk that you could get prosecuted for that) just to leave the client open, it's because people just don't care about it. There are so many things to care about in this life, so it's better to offer at least some reward if we want that people invest some attention to the network maintenance.
NXT is a little different than bittorrent in that the value of your NXT will drop if the network becomes insecure. If you're a torrent seeder, you're providing a service out of the generosity of your own heart. Additionally, the files your are providing don't actually have any invested value - it is more than likely most seeders are not the ones who created that file, so they could care less if the network failed. In NXT, you're providing a service (forging) because you want to preserve the value of your NXT. Due to game theory, the only node you can truly trust to act responsibly, is your own.
So think of it this way: if you had a multi-million dollar business built on-top of NXT, would you want the network to be weak and insecure? Even if it looks very healthy now, do you trust others to keep their nodes up 100% of the time? Heck no, you'd be forging to make sure the network stays up 100% of the time so it doesn't affect your business. Luckily, the cost of running the NXT client is cents per day.