TrackIr is a very useful little thing, even though not everybody seems to be convinced. So, to clarify a little bit, i'll post this.
As said somewhere above, it comes in handy especially in prone.
Right now you have to turn your entire body to look left or right, and as most of us know, movement is what gets you spotted most of the time. Reducing the amount of movement needed to get a good look around (by adding freelook) you only turn your head around, keeping the rest of your body still, which makes it a lot easier to remain unnoticed.
Considering remaining unnoticed is what helps you survive in RO, this would be a very cool thing to have.
Second, it allows you to keep your weapon pointed in the direction where you expect the enemy to be, while still being able to check the spots the enemy is less likely to be in but which still need to be checked.
This can save precious time in which to react, and thus survive.
You might think of this as an unfair advantage, and i'm inclined to agree to a certain degree yet the difference would only be milliseconds mostly, plus getting used to the trackir takes some time and effort as well and people who arent used to it yet can also be at a bit of a disadvantage during these quick-reaction moments because of that
And ofcourse, i'd also say that people who spend the necessary bucks on a trackir are ofcourse entitled to this little advantage.
You're most likely never going to notice it when you're playing against TrackIr users. It's not going to be like TrackIr users will suddenly completely rape everybody else constantly, and the TrackIr costs quite a bit for little advantage in a competitive point of view.
For me, it's mostly about the immersion it delivers, and the fact you dont need to press a button to look around. Other players could still go prone and look around with the press of a button and have the same advantage as the TrackIr users.
And ofcourse, it makes teamwork a lot easier because you are able to quickly determine the location of your teammates and adjust accordingly.
Which, i may add, will already be made easier on relaxed-realism mode by the addition of a minimap and thus also available to people who do not own a TrackIr.
The only thing that keeps me from using it sometimes is that i feel like i strain my neck to remain still and after prolonged use becomes a bit annoying, though i suppose this can be corrected by changing the TrackIr's deadzone.