Defensive shooting competitions can be used to train for real defensive situations, but you have to be careful how you do it.  The reality is that a defensive shooting competition is a game.  Because it is a game, there are some artificial measures of success (e.g. time), and people psychologically want to maximize their ability to meet the artificial goals.  However, if you are aware of the training pitfalls that will form bad combat habits, and don't mind handicapping your scores a little, you can get a fantastic training experience out of a match.

"You don't rise to the occassion.  You fall to your level of training."  Let's examine some things to watch out for:

• Don't think about the scenario ahead of time.  Before the competition begins, you will be instructed by the range officers as to how they expect you to progress through the scenario.  Pay attention to what they are telling you, but wherever there is an opportunity to engage several targets in the order and position of your choosing, don't plan ahead.  You want to improve your ability to think out creative and appropriate solutions on the fly, under stress, and the best way to do that is to start creating your action plan AFTER the scenario has started.
• Engage targets in order of threat to you (e.g. distance from you).  Don't get in the bad habit of engaging linearly left-to-right or right-to-left, even though this will give you the best time and score.  Engage the closest or most threatening targets first, then move to further away or less dangerous targets.
• Reload, clear malfunctions, and perform other actions from behind cover.  In a real gunfight, you don't want to be standing out there alone, just waiting for the enemy to shoot you, while you change magazines.  Take cover, to provide increased safety, while your ability to fight is degraded.  If there is nothing to get behind, at least take a knee to make yourself a smaller target and simulate taking cover.
• Take notice of the difference between cover and concealment.  Cover stops bullets, and concealment merely hides you from being observed by the enemy.  Even if the stage designers didn't intend it this way, don't hide behind a pallet (concealment), if a 55 gallon barrel (cover, when full) is available.
• Do not stay exposed from cover for extended periods of time.  You should only remain visible for a few seconds, when facing multiple enemies.  To do otherwise would allow them time to locate your position, and engage you accurately.  Those of you with prior Army or Marine experience may remember the "3-5 second rush", and the memory tool of saying to yourself "I'm up... he sees me... I'm down."  Pop out of cover, take a couple shots, duck back behind cover, and repeat.
• Do not pop out of cover from the same place many times.  Just like staying exposed too long, if you repeatedly come out of cover in the same location, your adversaries will eventually find you.  After using one location 2-3 times, move to a new location, use the other side of your barricade, or change things up somehow.
• Do not pop out of cover from the same place you entered it.  If the enemy just saw you run behind cover on the left side of a barricade, he will psychologically be expecting you to come back out there.  Crouch or crawl to another position, shoot from between holes or gaps in the barricade, or shoot from a different elevation on the barricade.
• Don't be afraid to use non-standing shooting positions.  Many stages do not dictate what position you need to be in while firing.  Most people use a standing position for speed, or because they just didn't think very hard about it.  Take this opportunity to shoot while kneeling, or even from the prone, to improve your steady body position and make yourself a smaller target.  You could also try laying down sideways ("urban prone") and shooting from under or through a hole in the barricades.
• Don't crowd the cover.  We psychologically want to get as close to the target as we can.  While this has speed and accuracy benefits, which play into the timed game mechanics, it could be bad in combat.  Stay a couple feet back from the barricades, so that you increase your field of view around them, and would be able to see and engage threats that came up around the wall.  You most certainly do not want your muzzle/barrel sticking out beyond the cover, where an enemy could grab it, though most competitions penalize for this.
• Return fire first, or provide suppressive fire to cover your movements.  "Suppressive fire is fire that keeps the enemy more concerned with not being shot than he is with shooting at you."  If you can present the weapon and fire a couple shots at a target while moving to cover, that hit your adversary, or make him take cover, you buy yourself time to use your sights for more effective follow-on shots from behind the protection of cover.  "Return fire, Take cover, Return fire."
• Bonus #1 for Law Enforcement / Military:  Use a proper stance to orient your body armor to the target.  Heavily twisted stances, like the Weaver stance, were invented in the competition world, and before body armor was prevalent.  If you are a professional that wears body armor in the line of duty, do not get in the habit of using a stance like the Weaver, that would present your unprotected arm-socket to the enemy.
• Bonus #2 for Law Enforcement / Military:  Wear your kit.  Most defensive shooting competitions will allow you to wear tactical vests, gun belts, body armor, etc.  Some may require that you be an armed-professional, though I don't know what proof they would require.  You may also be allowed to do it, but not have your scores count against the serious competitors (some highly-competitive people will become very poor sports if they perceive you have an advantage over them, such as being able to reload from pouches on your chest while they must load from the belt).  Train how you fight.