In the intro to Larry Hartsell's book Entering to Trapping to Grappling is the following statement, regarding the strengths and weaknesses of various arts. Bruce Lee studied the strengths and weaknesses of various arts and wanted his students to understand those for themselves.
In my opinion, studying the strengths and weaknesses of various arts through the lens of JKD, for some became confused with the tacit endorsement of said art. And for some it becomes a means to discredit those who study the strengths and weaknesses of those arts.
I believe that for some of those who don't understand the exploration of various martial arts to study their strengths and weaknesses and how we can use JKD to counter those. Some personal examples I've noticed regarding the arts listed in the forward... Are as follows:
Boxing is an integral part of Bruce Lee's formation of JKD, it is important to understand the fundamentals of boxing, whereas JKD has a more economical non telegraphic structure, understanding the fundamentals of boxing is crucial. The economical structure of JKD and the use of kicking, trapping and grappling can be used to counter boxing.
From Savate, I like the mobility and nimbleness, the use of the feet, and lead foot, the way that the boxing facilitates kicking. As a combat sport it is pressure tested. The use of trapping and grappling can be used to tie up a savate fighter who is nimble on his feet.
From Muay Thai I like the power behind the kicks, elbows and knees, and some of the sweeps out of the clinch. As a combat sport that is a lab to test what works, there is a reason that it's a part of a lot of MMA schools curriculum. Countering Muay Thai, using footwork to evade the kicks, stop kicks, trapping to shooting into takedowns into grappling and avoiding the clinch elbows and knees.
From Judo which is an Olympic sport, I like the application of throws against a resisting opponent, hit them with the earth, as we say. Judo is a pressure tested combat sport so there are functional throws that can be applied in the street and MMA. In the Long Beach demo you can see Bruce Lee "Osoto Gari" the outside reaping throw. Bruce Lee trained Judo and according to Dan Inosanto had a few throws from Judo that he liked. We know he trained with Nishioka Hayward. Countering Judo can be done with kicks, boxing, trapping depending on the Judo school they most likely have trained some ground grappling unless it's purely focused on Olympic style. You may want to avoid Grappling unless you're a high level Jiu-Jitsu player or grappler or use asymmetrical tactics and ground and pound.
From Jujitsu, you have a solid ground game of control and submissions. There is a Trapping to Grappling series that Bruce Lee worked with Dan Inosanto from 1966/67 that includes Throws, Takedowns, Arm Locks, Leglocks, Neck Cranks and Strangles. There is a core curriculum of 33 Grappling Techniques that Bruce Lee taught to Dan Inosanto and Larry Hartsell some of which is also in the Tao of JKD. From Small Circle Jujitsu, I like the application of the "small circle" principal, we know he trained with Wally Jay. Of course Brazilian Jiu-Jitsu application of "rolling" adds a level of pressure testing that makes it a functional combat sport that is a part of many schools MMA curriculum. In the Tao of JKD the section where it mentions the Do's and Don't of grappling it applies to grappling with resistance. To counter w JuJitsu practitioner using your kicking, boxing and trapping and unless your a high level grappler you may want to avoid Grappling or use asymmetrical tactics to get out of grappling back into kickboxing and trapping range, or throw into the ground and pound.
From Wing Chun which is also a core of Bruce Lee's art, I like the stickiness of close quarters trapping. Not being a combat sport the Internet is full of a plethora of videos of MMA fighters crushing Wing Chun fighters. I like to apply "functional trapping" out of an MMA platform. To counter Wing Chun you would want to focus on maintaining kicking range, long range boxing and shooting deep to grapple to neutralize their arsenal.
From Silat I like the use of various sweeps out of a MMA platform using sweeps such as Sapu and Biset have been added to some Judo players to improve their throws. I like to use the sweeps out clinch. Not being a combat sport does lead to a lack of pressure testing. Pressure Testing through sparring is crucial for a functional fighting system. I've not seen Silat used in a full contact fight situation but various throws and sweeps can be applied under pressure and add to the takedown game. To counter Silat a solid application of kickboxing and grappling would be useful.
From Aikido I like the use of wrist locks (which I find more useful in Grappling where it's easier to isolate a wrist for manipulation) and Tai Sabaki, using your body to create torque to throw a larger opponent. I like it being described as the "art of fighting without fighting" but not being a combat sport means a lot happens that isn't pressure tested with a resisting opponent so there is a plethora of non functional material taught. To counter Aikido just punch them in the mouth. They won't know what to do. I'm in joking well sorta. Most Aikido don't train out of a boxing / MMA style format. So kicking boxing and grappling will neutralize most of the traditional Aikido.
These are only some examples of studying strengths and weaknesses. Add your own. But remember it's not the art it's the practitioner that makes any art functional.
Studying the strengths and weaknesses for Bruce Lee, had more to do with developing a superior martial art. Pressure testing against a resisting opponent through sparring is something Bruce Lee was doing with his core group he was exploring his ideas of JKD with and is crucial to testing the strengths and weaknesses of various arts.
Studying the strengths and weaknesses isn't meant to "add to JKD" as some misunderstand and assert. And I'd agree that some people who see the study of strengths and weaknesses and misunderstand it as endorsement and that can be confusing but we shouldn't stop studying the strengths and weaknesses because of that. Bruce Lee still wanted JKD to be a superior art so testing JKD versus various arts helps us refine JKD in application to various arts and keep it functional and relevant and not stuck fighting fighters of 1960 when we live in 2023.
Being able to apply your JKD against practitioners of various arts by pressure testing it through sparring can help make your JKD applicable and relevant. Whether you call what you practice Jun Fan Gung Fu, Jun Fan JKD, Later Stage JKD, or JKD Concepts, or "insert name"'s JKD pressure testing through sparring is crucial.
To be clear I am not advocating style-hoping in order to understand JKD. Researching the Pros and Cons as Bruce Lee studied the Pros and Cons. It would be an error of judgement to confuse studying the "strengths" and "weaknesses" of various arts with style hoping but also it would be an error to teach other arts from "beginning to end." Those two points aren't mutually exclusive and can be utilized concurrently in tandem.
This quote is about Bruce Lee wanting his students to also understand the Pros and Cons or as the quote says "strengths" and "weaknesses." Bruce Lee saw JKD as superior and the study of strength and weaknesses was to ensure that JKD was responsive and relevant to "counter" those arts.
"Style-hoping" isn't necessary to help someone understand JKD. But let's be clear I'm not advocating for "style-hoping" so a caution someone sloppily slapping a bunch of martial arts together and calling it JKD, well that's not JKD but that error has occurred by those who don't understand. So cautioning against avoiding that is important. You don't have to "style hop" to understand JKD, that isn't how you understand JKD. JKD can be taught on its own and should be.
This is about studying the strengths and weaknesses and showing JKD to be superior through pressure testing through sparring to see the "pros and cons." If you understand those ideas are not in conflict.