Just because orbitz/expedia/travelocity show it as available does not necessarily mean that by time you hit the "confirm" button that you can book it. often they show the lowest fare that was available, but when they go to confirm it, you don't get it.

Second, the only on-line booking bonus I know about on NWA.com is for a first time booking. They have done away with the continual ones. If you have never booked on the site than you would get a bonus, otherwise there is no longer an incentive to use their site. Unless the $5 fee on the other sites is an incentive to you.