Before I can answer the original question, a few questions must be asked:
- Risk assessment of what "self-defense" is.
- Is the art being assessed the sole provider of self-defense in your regimen?
"What is self-defense" is something that has been asked a lot here. It could range from "an untrained unathletic person throwing a single haymaker" to any of the following:
- Multiple coordinated opponents
- An olympian athlete
- Weapons
- A skilled fighter with multiple black belts and professional fights in multiple systems
Or it could be multiple olympian athletes who are all 30-0 in professional fights, all armed with various weapons. Of course, I personally believe that a
reasonable assumption is that the
majority of the fights you're going to get into are going to be against untrained or moderately trained opponents with more athleticism than experience. And while I do think it's realistic for multiple attackers or weapons to be involved, you also need to have such a higher skill ceiling against them that I think it's unreasonable to train beginner or intermediate students for those situations (if you want to cover them at all).
This brings up further questions. Do you need to be able to defend punches like a boxer? Or just defend against a layman? Do you need both striking and grappling, or are you confident enough in one area that you feel you can keep the fight where you want? A wrestler or BJJ fighter may feel confident enough they can close in and grab against any striker. All it takes is one grab and the striker is done. On the other hand, a striker may feel that he can use his footwork to keep any grapplers at bay...maybe not professional-level grapplers, but he's not expecting to go against that. Or you may feel the need to be proficient in both arenas.
Then there's the question of whether your training exists in a vacuum. Maybe you cross-train. Maybe you have prior experience. You might think Krav Maga suffers from a lack of competition, but if you already have competitive experience, it can give you a different perspective on self-defense situations (something I feel sport arts lack). You may think sport arts suffer from that lack of perspective, but they can give you a different perspective on the effectiveness of your techniques than an art with less competition and/or sparring. Boxing has one of the narrowest scopes of any martial art. If you want to be competent on the ground and standing up, then boxing by itself is a bad choice. But, boxing mixed with another art is a great choice.
Then there are your goals. If you want to be a professional boxer, then time training Krav Maga or BJJ will potentially distract you from that goal. Say you have 20 hours a week to dedicate to training. That could be 10 hours of fitness training and 10 hours of boxing training. If you mix in a couple hours of wrestling, that's 10% of your weekly training budget going towards something other than boxing.
It's a very personal decision. What are your goals and fears? How do you prioritize them? What is your time budget? How effective do you need your countermeasures to be against those fears? I mean, the ideal training regimen is that you take half a dozen different martial arts and do a full-body workout (including stretching and cardio) every day. Ain't nobody got time for that.