For the purposes of this discussion, "seminar" here means an intensive training session, ranging from a day to a few days. The focus can be sharp -- one instructor offering specifics about a single art; or several instructors trading techniques from multiple arts.
Generally, I feel, students of a particular art get more from a single teacher in their system working from what they already know.
On a typical weekend gathering, something like this: Students and teachers arrive on Friday evening, do meets-and-greets. Sessions start early Saturday morning, run all day, different teachers leading the class, or, if it is large enough, breaking into sub-groups with teachers rotating through. When the Saturday session is through, students and teachers hang out, eat, drink, have a fine ole time, crash, and then crank up again Sunday for a half-day before everybody heads home.
An example of a larger one was the Las Vegas gathering for Guru Plinck's students in '08. I spoke of it here, and that included some comments as to what I found most and least valuable about it.
The gist of that report was that having a chance to meet and cross hands with others in one's art, or arts that are in the same general ballpark, is high on the list of good things. The camaraderie can be great. That guy you thought was an idiot when you read his postings on the net turned out to be okay in person. (Or vice-versa.) You get a chance to see people who have a lot of years training and who have put their spins on how best to do it, and how what you know works or doesn't work compared to that.
I think newbies tend to benefit from multiple-art seminars more than folks with a lot of training in one style, and that's because they don't have all that baggage to get in their way.
If you don't know anything about a subject, it's all new and you can get into it without a lot of unlearning. If you are several years deep in an art and you have internalized the principles, then training that goes against those principles tends not to stick. If you are a good student, you will listen to the teacher and make an honest attempt to do it the way s/he tells you to do it. You are supposed to leave your rank and sense of superiority at the door, but it's harder to leave your skills there, whatever they are.
A simple example: In Sera Plinck, we generally try to cover both high- and low-lines. One hand high, one low, one near, one far. Even if both hands aren't always positioned that way, mentally, you designate them so that both lines are theoretically-covered. The right hand might be higher than is optimal for low coverage, say, but you know that it's your low-line and have it ready to move where it needs to be.
Obviously there are going to be transitions where this won't be possible, and you hurry to get there, but the only time you leave a big opening on purpose is when you are inviting an attack, trying to draw it in, because you have something in mind.
If you are attacking high-line with a knife, and the other guy throws up an X-block, then everything from his armpits down is open and if you have any training with a blade, chances are good that you can exploit that. Plus you have your back-up hand. He'll be playing catch-up, and action is faster than reaction, so chances of him getting his hands down before you gut him or use your back-up aren't really very good.
So when you go to the seminar and a teacher gives you a technique wherein your low-line is left uncovered, you are apt to feel uncomfortable. You know that you could go there if your opponent offered it thus, and the temptation is to do that.
Another problem is in arts that are similar but not the same. The kali instructor tells you to hold the stick this way. The escrima guy offers his method, which is different. Either might work, but both teachers are adamant that their version is correct and anything else is wrong.
When I was starting junior high school -- called middle school now -- we had a study block called MAS -- music, art, and speech -- each of which ran three months. In the first music class, the teacher asked rhetorically, "What is the universal language?" Her answer was, "Music." Standard notation was the same everywhere in western music.
Music is the universal language. We all nodded sagely. Of course.
Later that morning, I had a shop class, drafting. (This was back in the day when the boys all took shop, and the girls, home economics.) So coach -- and all male teachers were called "coach" then -- said, "What is the universal language?"
My hand shot up. I know, I know! "Music!"
Coach started at me as if I had turned into a giant horsefly. "Music? If you are trying to tell a chinaman how to build a house you gonna sing it to him? Not music -- pictures! You draw him a picture -- that's the universal language!"
I came to suspect in later years that the music teacher and coach got together and set this up to make us feel stupid, but in that moment, I surely felt like a fool. And I remembered it all this time.
What is correct in the moment depends on who is asking and what s/he believes is the answer.
And listening to the kali guy and the escrima guy, both of whom were offering stuff that didn't really jibe with my silat principles? It wasn't as confusing, because I had long since come to realize that one size seldom fit all, but still, it is something of a conundrum to newbies when they hear the contradictions.
So while I tried to do what the various teachers offered at the seminar, and respected that their way wasn't necessarily wrong, just different, there were times when I just shook my head and kept my mouth shut, even when I was pretty certain that trying a technique their way would get me smacked flat or maybe killed. Based on my knowledge that if they tried it that way, I could probably smack them flat or kill them, and I'm not even very good at this.
It's generally not encouraging when you look around the room as somebody is showing a technique and you see students grinning behind their hands at what is being shown. Yeah? Right? I'll do that when hell freezes over, thank you very much.
I admit it: My teacup, while not altogether full, is, at this point in my life, not empty, and being able to empty it in this situation simply isn't possible. It took me a fair amount of time and practice to replace my Okinawa-te responses with those of Silat Sera, probably a couple of years. I know I won't have the wherewithal to do it during a weekend seminar.
There are people who see this as a major flaw. That I'm locked into stuff that will cause me more trouble than it will solve. They could be right, but I don't believe it. If an art is going to be useful at speed and power, it can't rely on conscious thought once action commences. You won't have time to think, so if you haven't built something almost reflexive, you'll be behind the speed and power curve. Either you figure out a short-cut or you don't catch up. Waiting to figure that out on the fly and in the moment doesn't seem likely to me.
And finally, the problem with doing a two- or three-day intensive session is information-overload. If you come away with one or two useful things, you are doing good. It is, as I saw recently on a martial arts website, like trying to drink from a fire hose. Plenty of water there, but moving at such speed and pressure that it will be hard to quench your thirst.
These days, I mostly confine my seminar experiences to those taught by my teacher, or as in the case in Las Vegas, others who have mostly been his students and who have branched out. Life is too short to learn everything ...