Basically, it's not the "best" method of fighting.
Boxing, Judo, Wrestling, Muay Thai - all have one thing in common: They limit the skill set you learn to a set of "safe" techniques, and frequently practice them full speed, full contact.
Training any one of these makes you a force to be reckoned with. A decent boxer has skills that will serve him well, a successful high school wrestler has skills that will serve him well.
IMO anything you train in will be successful if you engage in full speed, full contact application. You will find what works and what does not, and that will be that.
In all cases, people need to be aware of the limitations of what they do. A person who has trained straight BJJ at a competition oriented school for 5 years is likely to be deficient in takedowns and striking. This has been the downfall of many straight BJJers in MMA. Boxers often just assume they'll hit a grappler on the way in and that will be that, etc.
I'd say the biggest factor is not to kid yourself about what you are doing. For some, that means recognizing they are sportive grapplers, vice "fighters." For others, that means recognizing they need to add full contact, full speed sparring, etc.
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