Not allowing custom banners is similar to taking half the fun out of the game. Sometimes, people don't just play for domination and competition, they just play because they enjoy the medieval simulator and like the gameplay.
I can't speak for EU, but of the largest factions, GoW, MB, FCC, and more sometimes largely use custom fun banners(MB having 175 players, FCC having 161, and GoW having 116). That hardly looks like 10% of the population, and this does not even include the clans with less than 100 people in them.
Besides, we NA people like holiday banners too! Would almost all of our banners be blacklisted if we used I.E. Chaos for an example, spooky tree during the holiday season?
Edit: and BIRD 105 players(now inactive) Pink Wing with B
