At my current dojang, we've had some visit (I haven't been there long, though). I think it's good to visit and have others visit, too. However, I was just wondering if anyone has had any negative experiences with this, and wanted to know if it was becoming "taboo" to visit nowadays... Personally, I've only known of one local instructor (not affiliated to where I go) to put other instructors down, which did sound a bit like jealousy to me...
I haven't had any negative experiences, per se. I have had some "WTF?" moments but not necessarily negative.
Once while visiting a school where a friend of mine trained the instructor started bad-mouthing my instructor. Not blatant insults just insinuations. I told the guy, "Look. I'm here to hang out with my friend and get some training in. You've never met my instructor so nothing you say is going to have any bearing on my perceptions of him. You're just parroting stuff you've heard from others - and you have no idea what kind of agenda they might have. Why don't you come visit my school and watch or attend a class with my instructor. Then you'll have something to base an opinion on. If you still feel he's somehow lacking, fine. You're welcome to your opinion. If you say anything else about my instructor I'll leave." He didn't and we finished the class without further problem. Ironically, aside from being an overly impressionable follower who was too quick to toe the political lines that his instructors wanted, he was a nice guy and a decent martial artist.
Last year I visited a school out in CA. I was in the area visiting some friends and noticed the school. I looked up their class schedule online and showed up about 15 minutes prior to their class the next evening. I approached the instructor and explained that I was in town visiting and, as a fellow Silat practitioner, I had heard of the system they taught but I'd never seen it firsthand so I wondered if I could participate in the class or at least watch. The instructor was obviously a little reluctant but let me participate in the class. Afterward we talked for a bit and she said, "I was a little worried about your intentions but you were very respectful and I appreciate it." I asked why she was worried. She said, "Well, we don't ever visit other schools or instructors and it seemed a little strange to me that you'd want to." I asked why 'they' don't visit other schools or instructors. She said, "We consider it a sign of disrespect to our instructors and system."
Personally, I've always thought that was a little strange. I also think the idea that someone can "steal" students is somewhat warped.
In my personal experience I've met quite a few great martial artists. I haven't trained with any of them. My reasons for not training with them had nothing to do with their skill/ability/personality. They were great people and phenomenal martial artists. My reasons were purely personal and, more often than not, it boiled down to the fact that I was completely happy with my instructor and training.
Since I know this from personal experience, I don't take it personally when someone doesn't want to train with me. There are a lot of reasons that someone might choose to train with someone other than me - and none of them have anything to do with anything I'm doing wrong or lacking.
People are going to train with whoever they want unless that instructor turns them down. No one can "steal" my students. If they choose to train with someone else, so be it.
Same goes for "stealing" techniques or material. So what if someone comes in and learns a technique from me. Unless they put in the "dirt time" to develop that technique for themselves then it'll never amount to anything more than a parlor trick in their tool box - if it gains that much status.
Anyway ... I've rambled long enough

I'll put my soap box back in the corner.
Mike