I have a viewpoint here that I think others may disagree with, but having heard the same from Bruce Lee's personal student here it is:
Jeet Kune Do is an individualized martial arts system developed by Mr. Bruce Lee in 1967 for development of and for use by himself. The system has its own definitive stance and techniques, referred to as "tools", used for each perceived type of empty handed fighting. Heavy emphasis is placed on mobility and physical development for functional use.
Each tool developed is designed to be a primary tool used as a weapon in many varied scenarios. There is not a wide array of techniques used in jeet kune do, but knowledge and familiarization with other martial arts systems is encouraged, especially to be able to counter such fighters from other systems.
Instead of working with "forms", perceived as tantamount to a form of classical art or dance, emphasis is placed on the scientific development of "attributes", which include things such as speed, power (by velocity), strength, ease of movement, rhythm, and cadence (mental perception of speed). Sparring is practiced a lot to develop proficiency. When a certain degree of mastery is attained with the sparring exercise, sparring may be practiced against willing participants from other systems in order to expand one's experience.
After immersion in the conditioning, sparring, and expansion phases, the practitioner eventually returns to the practice of the basic primary tools, footwork, and further development of attributes. What was once complex is simple again despite the various forms out there, except you can do it better than most anybody else.
Having said all that about the training process, jeet kune do is still its own system. The same training methods may be employed with pretty much any martial arts system, but to say you "employ" JKD with another system like karate or jujitsu is not entirely correct since the essence of what it is at that point in time is redefined from the tactical system of JKD to the developmental process of JKD. By the same token, mixed martial arts is not JKD.
If you read those volumes compiled by John Little, there is a memo handwritten by Bruce Lee explaining to one of his students that mixing martial arts together is not the same is the simplifying and re-simplifying process of jeet kune do. It even has an algebraic equation on the page describing it.
For example, these days I personally practice a contorted system of defensive tactics used by police, security, and military personnel that uses tools such as compliance holds, basic grappling holds, handcuffing, and weapons, but no striking. This is supplemented by footwork similar to what I practiced in jeet kune do, trapping hands from Wing Chun, grappling methods from jujitsu courses I've taken, but still retains the development of scientific attributes from JKD. I suppose this would constitute a mixed martial arts system, but it is no longer jeet kune do. It does embody the "jeet kune do experience" though if you want to think about it in those terms.
Jeet Kune Do was unique to Bruce Lee in its true sense. Still, there are many things Mr. Lee wrote about during his lifetime that other martial artists can develop and learn from, even if that person's style or system of tactics is totally irrelevant. This is something the whole community of martial arts has definitely benefitted from as a legacy.
In closing I would like to mention something I feel strongly about. Something Mr. Lee told all his students and is in his writings as well is the statement "Jeet Kune Do is just a name. Please don't fuss over it." He felt that by creating a thing that was so dynamic for the time he would be unifying people in martial arts circles, clarifying differences, and coming up with a means for people he knew at the time who were in closed communities in the United States to successfully integrate and become more prosperous.
Since his death in 1973, people have done just that though. I see endless bickering about what jeet kune do really is/was regardless of whether you were personally there to see it or not, which techniques are better, arguments about Mr. Lee's personal life, and countless other things that don't amount to a hill of beans in the real world. Some folks are even exchanging death threats over this kind of crap. This was the main reason I left a jeet kune do school ten years ago.
Some statements Bruce Lee said or wrote included "we all only have two arms and two legs. There are only so many ways we can use them." and "after all, we are all God's children under the sun" (or something like that). Statements like those are sage advice to live by.
"After all, we all have only two arms and two legs."
"Death is certain, life is not."