I'm not sure about specific defensive training, but I sure did learn about teamwork and group tactics playing Tribes 2. Since it is most commonly played in CTF and the best servers were capable of 60+ person online play (no bots), you had to communicate and know your job well. If you didn't play as a team, you simply lost. Also, the physics of the Torque engine were superb. Things like gravity, wind shear, and momentum were calculated into everything, from weapons fire to vehicle navigation to simple walking. Unlike games of the same era (such as Quake 3), a missile fired when traveling at high speed did not track along in a straight line from the point of fire to POA. You also had to be wary of firing mortars into high winds - I blew myself up once or twice trying that one.

Projectiles in the game also had more realistic rates of travel and arc of trajectory - you had to lead your aim on a moving target and adjust your aim for distance. You could also hurt or kill yourself if you jump from too great a height. Increasing levels of armor also progressively slowed you down (unlike most FPS games). You had a finite supply of ammo, and a reload station was not always nearby. It was truly and underrated game, probably because of the high learning curve due to its more realistic physics and gameplay.
Some valuable things I did learn playing from Tribes 2:
In combat, keep moving. A moving target is much more difficult to hit.
Learn to shoot the weapons you carry. A skilled shooter has a distinct advantage over someone who sprays and prays. Take enough time make your shots, or you're going to miss. Doing this with speed comes with time and practice.
Cover is your friend. Moving behind cover gives you a tactical advantage and the element of surprise.
Be aware of your teammates and what they are doing. Communication and a plan of attack are key to everyone staying alive and your side winning.
Well-placed surveillance equipment is a big advantage, and can help you get the drop on the BGs.
Distraction and surprise are effective combat tools.