I like (and trained in) JKD because it emphasizes street fighting (real defense, not competition) and not going to the ground, which is a terrible idea on the street but works great in a competitive fight with a soft floor and rules. San Soo (a form of kung fu) is also excellent for street defense (though hard to find instructors for) and what I started with with I was a kid.
You don't see JKD or San Soo dominating MMA because almost everything we trained to do is illegal in competition. If I were to get into the octagon and fight a trained MMA fighter according to the rules then they would probably hand me my ass real quick as most of my reflexive responses are against the rules. But when the rules don't apply then my chances of winning are very good. There's no hold I can't get out of fairly easily unless it's one that also immobilizes my adversary.
That is, if someone puts me into a hold I can't get out of (good luck getting me into it in the first place!) then that means all they will be able to do is maintain the hold until they get tired. They won't be able to make me move somewhere else or really do much of anything (they could still injure me by overextending whatever they're holding, etc.). But the moment their grip loosens so they can do something else it's go time, even if that means tearing off large chunks of their flesh or an ear or something with my teeth, or ripping their junk off with my free hand.
I also did a lot of tactical knife/stick/firearm/multiple adversary training which aren't really emphasized much in MMA but are important to be ready for on the street. I suspect that trained fighters are sometimes defeated on the street even by untrained meatheads because they lack the willingness to inflict truly horrifying brutality upon their attacker. Most self defense schools under-emphasize this since it's awkward to discuss and advocate such brutality or to intentionally develop a "killer instinct" but survival can be a very nasty, brutal business.
Granted, someone who trains BJJ, or for that matter any martial art to a high level will be capable of quickly defeating ~95% of the people they're likely to encounter in a self-defense scenario but for that last 5% you need a little more than what traditional or competitive forms offer (in my opinion and experience). Also, if a real life-or-death street fight lasts longer than 2 seconds then you're doing it wrong.
I got to where I was an assistant instructor for a while and would advise prospective students to explore an MMA-oriented gym or school instead if they were interested in competition since the habits ingrained in one context aren't ideal in the other. IMO we need to pick one (self defense or competition) and focus on it. That said, it's good to train BJJ anyway just to understand what you'd be up against and how to counter it since so many people train it now.
Just knowing how to sprawl is very helpful; if someone shoots and you do it right and slam them into the ground as you land on top of them it hurts them a hell of a lot worse (and might take all the fight right out of them) than if it happens in the octagon or on a soft, squishy gym mat. Granted, there's also a lot of street-specific stuff in BJJ that is illegal in competition but if you train for competition you won't habituate those techniques and so are much less likely to use them in a real fight (since we fight like we train).
Also, JKD still works great when you're older and not so strong anymore. That's not really the case with grappling. I've never wanted to end up grappling with someone on the ground, even when I was young and invincible!
Much of JKD was based on Wing Chung, which was developed by two women as a way to defeat larger, stronger attackers (and one General in particular who was being a real asshole). Or so the story goes, but it does seem plausible once you start learning it. Besides Wing Chung, JKD was also based on elements of Western boxing (a VERY effective marital art) and European fencing (for the footwork).
That said, it's real hard to find GOOD instructors of JKD. Many muck it up by adding stuff to it which is the opposite of what Lee was trying to accomplish in creating it (but adding to it feeds their ego, I guess).
That's my two cents on the subject, for whatever it's worth.
