I think one of the big differences between the mindset of MMA & JKD vs other more traditional arts is in WHO is allowed to do the mixing. MMA and JKD pretty much state that every fighter needs to find there own style, draw from whatever they can make work.
That's different than what some of the other MMA proponents say.
You say:
draw from whatever they can make work.
While THEY said only BJJ, boxing, wrestling, BJJ and Muay Thai can be trained and be "MMA."
Ok, another analogy:
If "what works" = hot, fresh food,
Some people seem to believe think the ONLY way you can get hot, fresh food is if you get it from a buffet
presumably because THEY were served cold, old food from a restaurant.
But who is to say, if you order from a GOOD restaurant, that you can't get something off the menu that has everything you like, hot and fresh?
And who is to say that eventually SOME buffets will begin getting lazy and letting their buffet food get old and cold?
Once again I maintain:
No one system, school or organization has the ONLY source to good techniques and solid training techniques.
The more popular MMA organizations get, the more likely you are to get students who are in it "just for the health benefits" and don't WANT to get hit hard and instructors who are willing to water down the combat applicability until you get -- SURPRISE! -- MMA-trained students who are ineffective in real combat/self-defense situations.
Generalizations are dangerous. Just because your local Chinese restaurant has lousy food doesn't mean the Chinese restaurant in MY neighborhood sucks.
Just because your local Mexican buffet has great burritos that fill you up doesn't mean I am going to get a great, satisfying burrito at MY local Mexican buffet.
Nearly all restaurant chains start out with a great reputation -- which is how they are able to sell franchises. Eventually some branches of a restaurant chain let their standards slip (hard for corporate to keep a close eye on hundreds of franchises) and the reputation of the whole chain begins to suffer.
Those with a vested interest will stand on the mountain tops and scream that it ain't so, but time will prove me right.
So see past the marketing efforts, use your eyes and your head and pick what is best WHERE YOU ARE.