I am in wholehearted agreement that there is alot of bovine manure out there in the martial arts world, and very few teachers that can really walk the walk. However, i would not be as quick to throw out ideas that have been passed down through the traditions we train just because you, or your teacher cannot make them work. You know that in the old days, this stuff really mattered and you gotta ask yourself why would a family or group pass down information that might get the next generation killed? What was it there to train?
One of my teachers has a great story about when he was working in the mental health services and had to perform a takedown on a violent patient. As he is throwing and restraining the guy, he realises that he is doing a technique that HIS teacher had passed down to him as "part of our tradition but i don't know why, as it can't see it ever working".! As i remember it, he noticed that the foot placement, or the angle of entry was a little different from how he had learned it ,but that was all the adjustment that was needed to use the technique in real life. After this, his teacher changed the teaching manual to take his adjustments into consideration.
Sure, information can get watered down or distorted over the generations, but i think many concepts, like your example of trapping, are totally valid. The bigger questions for me would be, "why won't this work for me/my teacher"? , "what's missing"?, "how/when/why would this concept work"?
If a knife technique only works if the knife is held out front and frozen in space for a second, can i create that moment in my opponent and insert the technique by being in the right place at the right time? Perhaps, perhaps not, but it's worth investigating.
My personal favourite is checking the knife hand. This will get your hand cut off 90% of the time, but you know, 10% of the time it is an absolute necessity, and can work very well IF you do it at the appropriate moment.
As for playing with people from other systems,my teacher Sonny Umpad (who by the way could walk the walk), said that the one that hits you is the one you don't see coming, so yes, cross train as much as possible, but at the same time, IMHO it's also good to spend some time looking at the ideas within the systems that you train seriously to see what might be useful.
Side note to CWS, and others. Thanks for aknowledging that i am indeed a "gal". For simplicity on the forum, without having to go into the whole him/her, he/she thing, i reckon i could be an honorary "guy" if thats ok with you...honestly i've been including myself in the "guys" group for years now...martial arts kinda does that, so i'm not taking offense here