Marching band field shows are every bit musically and artistically important as large forms of "serious" music. I would even go as far as to say, that in the modern era, especially in North America, these field shows communicate ideas far better than traditional forms such as operas.
I recently had the opportunity to go see the Calgary Stampede Showband perform their field show "Rush" and I've linked to it below. I will be referring to this show when I make specific examples. And I do think this show is something worth your time to listen to, it's just under 10 minutes long, and the performers are so very engaging, that time will just fly by.
I hope you can agree with me that the entire point of performance, any arts performance, is to convey ideas from the performer(s) to the audience. Keeping that in mind, I don't think classical music does that for the North American audience. Communication is often blocked by external factors that don't involve the actual music. Take opera for example. How could an English-speaking audience come to understand songs that are being sung in a foreign language, let alone relate so deeply to them as to make up for the ticket price? I for one can't do that, especially reading the surtitles. Opera companies tries very hard to, at least I assume to make up for the communication gap by using surtitles and such, but in the end it just takes away from the enjoyment of the music. And we cannot forget symphonic works that require long attention spans and a vast musical vocabulary to fully enjoy the work, to actually take in what the performers are trying to communicate to the audience, just for their efforts to be thwarted by lack of interest and attention.
No, to communicate with a North American audience, these forms need to be changed somewhat to align with what these audiences are used to.
The first thing that might surprise you is the use of popular music. I think that with the use of "tunes" that the audience already recognises, it solves the entire problem of relatability. The audience won't become bored easily by something that they already enjoy and are familiar with. Take a listen to the first tune of the "Rush" show, "The Ecstasy of Gold" from "The Good, the Bad and the Ugly". Even without any visual cues, you can instantly put together a picture in your head on what this show is about. Images of the west, guns, and cowboys perhaps? It's a great use of music to set the scene, where the overture in an opera would be, it doesn't leave the audience grasping at "what the composer meant", but instead puts them right in the middle of the action. I can hear you now asking, "There's no original music? I thought the entire point of going to a concert was to listen to the music?" I think that if the point was to listen to music, there would be no problem with large forms such as opera, since audiences going to those shows would already be familiar to the music presented. But the reason why this marching band show so caught my eye, or ear, is for the very reason that it appeals to people without the background of classical music, and it effectively communicates an idea that a larger scale work such as this show would be. The use of popular music is its selling point. And I would remind you that the whole is greater than the sum of its parts. The chorography and dance in marching band shows are what tie the music and the story together. The music might set the story, but the dance makes the show.
I like to think about it as such: the musicians are the "chorus", while the colour guard (dancers) act out the important parts of the story. While in an opera, the chorus would use words that the audience cannot make out, or the orchestra would play some cryptic motif or "musical symbol" to move the plot along, in these field shows, the band plays tunes that immediately set the scene and move the plot along because the audience members are already familiar with the repertoire. By removing the "block" that is unfamiliar music, the musicians and the colour guard can not only communicate with each other on stage, but also very easily with the audience.
I can tell you now, that watching this show was an experience. Something that I don't get to feel much while watching opera. The lack of words didn't feel like a barrier at all when it came to communication and the fun "pop-esque" music moved the story quickly to avoid boredom. No one fell asleep, no one had to read the libretto before attending, no one left during the intermission, and yet there was still a standing ovation given. Perhaps it's time we changed the way we think about art music as a whole.