Fair is good as a quick hit, and if your opponent is out of jumps, or you read him well enough, you can get another one in. It is also your main move off the ledge, as a tipped one sends a horizontal angle. It's easily Marth's best move, IMO.
Nair has higher knockback tipped than I think all his aerials. (Although, tipped, all of his aerials are stronger than they probably should have been for a quick character like Marth) What you have to keep in mind, is that if you directly use it out of your short hop, you'll go over peoples head. You really have to time it. A good thing I'm thinking of for uses are nairing so they're off the edge, then fairing them for a kill. Although, I'm pretty sure Nair knocks your opponent diagonally up.
I've seen nairs chain on people after it's knockback has been staled. I've seen fair do it as well, but I'm pretty sure the hitstun is too low, and people would be able to get out of it if they knew how.