I think much of this is very cultural, and regional even. For example the match at FA between Jean and Nathan was quite raucous and a lot of the crowd was very much into that match. Lots of chanting and cheering which is not really any different than a boxing match might be here in America, or most any other sporting event. I can't speak for Nathan, but Jean didn't mention being bothered by it at all (in fact he mentioned one particular chant by our friend Christian helped him to get into the mindset he felt he really needed to be in for the match.) But I also noticed such chanting from the Swedes and Mexican groups as well. All supportive and encouraging, none insulting (and, indeed, they all clapped vigorously for great hits even when they were against the person they were cheering for.) It seemed to me that the if anything the cheering (and not jeering, let me make that perfectly clear) made the fights more exciting and provided the feeling that the spectators were really attentive and involved with what was happening. I think it raised the energy levels of not just the spectators, but of the fencers as well (and in a good way, mind you).
As for coaching, that also is done in the middle of many sporting events here, at least. Certainly during boxing matches and the like. As mentioned above it is often difficult for the fighters to hear such comments anyway because of their focus on the fight, but I don't see that they are inherently discourteous. Then again I personally find individuals who write in absolutes to be fairly discourteous in the culture I was raised in, but I have done a lot of travelling and long had a great appreciation for the variety of cultures around the world and, as such, I prefer not to take offense at such things. Writers on the concept of honor in 16th century Italy were known to remark that an offense can only be meant if it was intended as such and that it was not appropriate to take offense at, say, a Frenchman kissing the lady of the house in Italy if the Frenchman did not know that was not the custom in Italy, as it is in their own country, and an effort should be made to determine if an offense was truly meant rather than assuming it was.
In this way I think that it would be wholly appropriate for a particular event to list that they have a preferred "code of conduct", if you will, where they might mention if such behavior as cheering or a yelled coaching tip from the sidelines is appropriate. (Keep in mind here, that a yelled tip from the sidelines may also be heard by the opponent of the person being coached giving them just as much opportunity to respond in a way that would counter that advice.) I would very much prefer seeing something like that left up to the events themselves, however, rather than some formalized code of conduct across the board. I think it is those cultural differences that actually make us stronger as a community and I would like to be able to respect all of the differences we bring to the HEMA table.
As for staff usage, I also very much agree that I like to see it used by referees at tournaments, for all the reasons mentioned. To me the job of a referee is far more about safety than it is for just stopping matches and coordinating the results of the judges for a scorekeeper. In that sense I think there are a few other things we could talk about for how we should go about doing that, as referees, but that's really a topic for another thread.
Just my 2 cents, though, and we know how very little that is worth these days.