The most common form of armor for both legionaires and auxillaries has always been chainmail throughout the entire history of the late Republican and all of the Imperial era.
Auxiliaries and regular legions actually wore the same armor most of the time. The triumph statue commemorating Trajan's conquest of Dacia, the Tropaeum Traiani, shows both regular legions and aux wearing chainmail and scale armor. (and this was during the height of the empire when the segmentata's use reached its peak)
As for segmentata vs chainmail: Yes, it was a slightly better form of armor, but the cost did not justify the marginally increased protection.
1. The Romans actually didn't have a very good standardized metallurgical system. They weren't able to mass produce bands of iron plates. Creating a long wire wire and bending them into chain links was far cheaper and easier.
2. Repairs for chainmail is also easier - all you need to do is rivet new mail onto the shirt and remove the torn/damaged mail. For banded plate armor, you basically need to take the entire armor apart to repair it.
3. Also, the LS's copper joins and leather straps that held it together wore down very quickly. The LS's iron bands also rusted and was hard to maintain...whereas mail's friction from moving usually kept it rust free naturally.
4. Finally, segmentata had to be tailored to fit each individual. Even then, modern reenactors in LS have testified that it is still extremely uncomfortable to wear and causes chaffing. Chainmail however, is "one size its all" and is much more flexible and much easier to wear.
5. Chainmail is also easy to put on - whereas putting on LS took a long time and actually required another person to help you.
6. The Romans actually had heavy riveted mail with shoulder pads and sometimes extra layers over vital points. This heavy chainmail also provided groin and lower body protection (whereas the LS only provided torso protection).
The lorica segmentata didn't provide much better protection than chainmail, and was costlier, which is why the Romans ditched it.
Even if a Roman legion unit was wearing lorica segmentata, the centurion wouldn't be - he still wore chainmail and hung medals from his mail.
And the Roman soldier in the ads wearing the LS armor was actually a Praetorian guard (the crest on his helmet went from front to back, not side to side like the centurion).