Sorry, where was the strawman?
I suspect the people who create their flags are creative and expressive in various other ways in their lives. Whether that be artistic, joking with their mates, making memes, making music, or whatever. The best thing about Glastonbury is that people can be creative however they want, with flags, funny hats, fancy dress, dancing, face paint, or whatever they want, without feeling self-conscious. Maybe the average accountant or police woman or builder can't wait for Glastonbury every year, because they can let their creative hair down in a place where they don't have to conform to their normal peer groups' usual social acceptabilities.
Actually, MrZigster complained that they were all narcissists and show-offs. Which are extrovert traits - this seemed to be his secondary argument after them getting in his eye-line.
Maybe, just maybe, not all extroverts are the same? People express themselves in all sorts of ways - maybe your mates think to themselves "I'd love to take a flag this year, but Yogurt on a Stick might get all uppity about it". I don't know
And people have paid money to go to the Glastonbury Festival of Contemporary Performing Arts. Nowhere in the rules does it say that the performers have to be on a stage and as we always say in the Glastonbury eFestival forums, "It's not all about the music!". It's a place for the punters to "perform" as much as the musicians, artists, comedians, acrobats and theatre groups.
And after asking myself "Do I or do I not give a fuck whether I piss other people of, or not?", I came to the conclusion that it is not for me to tell someone else how to express their creativity. Banksy does it by vandalising buildings with spray paint. Damien Hirst does it by putting thousands of flies in a glass case with severed cow heads. Glastonbury punters do it by making flags for the amusement of their mates. Whether I like it or not, is not important.
At the end of the day, there are plenty of places where you can go and see your favourite bands without flags. They tour all the time in venues with tiered seating and big screens. You are going to a festival. Flags are synonymous with festivals of all types. Music or not.
Edit: some words.