Kohai raised a good point about Trigger Discipline, but I'm a little bit puzzled that its so much more popular than Fast Shot. Look at it this way: the longer an enemy is alive the more damage you take. The faster you take them down, the safer you are. I agree TD will be better early game, but I still think FS might be better after like level 15 or so.
I think it might be because of the following thought process for the two traits.
With Fast Shot you shoot faster but it is at the expense of accuracy, so while you may put more rounds into the air at your targets, each shot has a reduced chance of actually hitting the target.
With Trigger Discipline your shots are more accuract at the expense of speed, so you don't put as many rounds into the air at your target but each shot has a higher chance of actually hitting the target.
Basically I presume the concensus is that it is better to take a bit more time between each shot and thus have an improved chance of hitting with each shot than to just let rip aka Spray and Pray (I admit the comment I'm using for Fast Shot in the summarisation is probably biased).