Bullshido is a martial arts site devoted to sports martial arts practitioners, with a strong bent against unverifiable claims and hack instruction. They have a particularly interesting FAQ which touches on why they think sport/competition training is superior to many kinds of "street" training.
More in the extended.
The part of the FAQ dealing with competition versus "street" training
They raise a number of interesting points:
1) Practitioners who train full-contact with appropriate safety (via tapping out or pads for full-contact striking) can test their training by evaluating their performance in fights, be it sparring in class or competing.
2) Training against a resisting partner who also wants to win is very different from going through a drill, even a drill intended to model a realistic situation. A partner who is trying to beat you is probably a better model of a "real life" situation, anyway.
3) Non-sport combat training can cater to a fear mentality. Interesting quote:
They are still acting out of fear driven behavior. Without any knowledge of the moral and legal ramifications brought about by their actions. The truth is, they will never overcome their fear because they never confront it. Never testing their skills so as to give themselves a realistic view of their abilities. The fear compounds as they know they are untested which leads them to avoid the testing process for fear of losing and the cycle never ends.- Paul Sharp
4) It's unreasonable to imagine that a competitor who can beat you handily when rules are in place has no chance of beating you in a rules-free situation.
This makes interesting reading for me, because it would never have occurred to me to argue that "sport fighting" training does not improve ones real fighting ability, but apparently a number of people make that claim.
This does accord with the disdain I saw for BJJ and other ground-fighting training in a drill-based fighting class I participated in for two years. At my request, the instructor once showed what he'd do if I had his back -- it involved trying to twist my ankles, more or less (which works modestly if I'm barefoot and as unmotivated as I was that day, but won't work fast enough to keep me from choking him into unconsciousness).
The instructors and most devoted participants in this class also expressed the belief, discussed in the FAQ and linked documents, that a "real fight" is over in seconds, or you've done something wrong. This suggests a dangerous inability to recover should something go arbitrarily wrong -- for example, if your foot slips. I also recall in class discussing a concern I had about an attack meant to disable a bat-wielding opponent by striking his arm before he could swing. The instructor countered my suggestion that I was not fast enough by saying it's not an issue of speed, but of timing. I let that one go, but I also think it's untested BS. Timing is important, but the ability to cross two meters of open space to get inside someone's weapon reach before they can strike you is very much an issue of speed. I bet if you tested this by instructing the bat-wielder to swing only after their opponent began to close, you'd find a clear distribution of outcomes (hit, not hit) based on speed of the unarmed person (heck, this could be tested with an automated lever arm if you really wanted to standardize it).
In retrospect, a lot of the most accomplished people in that class might find themselves in trouble in a fight unless they ran a parallel reality check, as my favorite intstructor did (when she admitted that she'd basically just punch people in a fight, eschewing all the more complex combined drills).
I am, at this point, suspicious of anyone who suggests, as many of these folks did, that crosstraining is pointless or detrimental.
...and now I miss BJJ.