I think the danger is in looking at the nomenclature almost as brand names. JKD appears to be more selective in what its senior practitioners take from other arts and methods, in fact I would venture that not just 'anyone' can take something from elsewhere and still call their JKD 'JKD', because 'something' still has to define JKD as JKD and set it apart from other modern arts.
Also, whilst MMA overwhelming screams 'SPORT', it is not as simple as that. Looking at interviews with the UK and rest of worlds top martial arts teachers, there seem to be a lot of them who teach what they now call MMA, regardless of what the original background was, but that they tailor the material and training methodolgy depending upon what the student wants from their training, and will adapt to the professional cage fighter, law enforcement professional, military personnel, and those simply looking for 'high percentage' (of success in application) self defence methods.
It is also true that not only are individuals seeking out this kind of approach, but also, probably because of the high public exposure of cage fighting, so are professional bodies. And because more and more young men will be seeking to emulate their UFC heroes late on a Friday night whilst full of beer, cocaine and bravado, it isn't such a bad idea for the police and those of us who venture out as civilians, to know how to deal with double leg takedowns and being ground and pounded, (to use their parlance).
You could (and I believe I did) argue that the FMA aspects highlight most of the differences between what is [generally] defined as JKD or MMA, but then we have the Dog Bros, who mix it all up and confuse matters even further. It occurs to me that JKD practioners have a venue with Dog Bros, which may mean that they really need not hold back on much of their material and still test it in a live, fairly dangerous but rarely seriously injurious, manner. And it is not necessarily a young man's game either, looking at some of the participants, including the Bros themselves, and this is another important factor in the 'Vs' argument. Sports martial arts are almost entirely the domain of young strong fit able bodies males, whereas martial arts for self preservation are often sought and practiced by those who do not fall into this category but don't want to curl up and hope for the best if they are assaulted on the street.
Not going down the old argument route of MMA sports not allowing eye gouging etc, because an MMAist is more than capable of throwing the rulebook out when need be, and many will not be dissuaded by a poke in the eye in any case, but it is the nastiness of real martial methods for use in life threatening situations, that even up the odds and make the difference between sport and combat arts, even though there are people who would do well in either camp. Real martial arts is about doing whatever is necessary and escalating the level of violence to negate the threat, and staying ahead of the game by learning to deal with whatever is currently 'popular' in a simple effective manner. Most of the time, looking at some of these MMA guys, my method will involve apologising profusely, offering to buy them a drink, and then disappearing as quickly as possible, probably without buying that drink.