The giant leaping stab on the queen was awesome.
It has nothing to do with ninja/samurai unless a person places that construct on it. Sure, Anderson may have liked the "ninja" type stuff, but I doubt he sat there and thought "hey, if he does this huge jumping stab, it'll be just like what a ninja would do." As far as my understanding of ninjas goes (and it goes far enough), they don't really DO that kind of thing...
It was put in there because it looks cool, not because it is "ninja" like or whatever else you want to call it.
Also, postulating that the Predators have devised a number of fighting systems (read: martial arts to most of you) is not far fetched at all. Especially given the fact that the majority of their kills are made up close with wristblades and other melee weapons. If you are going to be fighting a dangerous animal up close, you had better know how to use your tools efficiently and effectively.