It depends on the competition. Slow-fire bullseye shooting may not have direct defensive application. You're taking your time, controlling your breathing, etc. in a real life-or-death situation you won't have that luxury of time. On the other hand, action-type shooting can be very benificial if approached with a defensive mentality.
I compete in both IDPA and IPSC 3-gun. I use IDPA to hone my defensive skill set and 3-gun to play with my toys. In either case, they are both better than plinking targets at an indoor range for combative training. Shooting fast AND accurate are encouraged, you're drawing from the holster/concealment and adding the element of movement and cover that you don't get at an indoor range. I don't see it as an end all beat all training situation. You don't have fire coming back at you (suppliment your training with paintball or airsoft)!
It really depends on your mindset. if you approach the competition as a game than you'll pickup bad habits like sacrificing using cover for a better time, etc. If your mindset is that its an opportunity to practice defensive skill, then you may sacrifice a good score to do whats right.
If you have IDPA or IPSC at your local range give it a try!