My first thought was how do you know it’s only for fashion? But, I also see what you’re saying. I see a lot of tack used that’s not really necessary. I know several 10-13 year olds eventing BN with 5-point breastplates, eventer watches, super matchy outfits, and micklem bridles just because they want them and their parents oblige. I think only one has a runnnig martingale in that mix. Meanwhile, my horse goes Training with no breastplate, martingale, or even studs, and a cheap watch with big numbers from Walmart. The extra stuff on the kids’ ponies does no harm and hopefully they’ll take care of it and have it around if they ever really need it in the future.
I switched to a figure 8 on my strong horse because I thought it would work better with her curvy jaw conformation, but saw no big difference (but kept it because it does really fit her face better than a flash). I bought a used boucher to try on her for dressage, but noticed no difference. I stuck it on our pony’s bridle, and it made a huge positive difference for her! I don’t know if it was the boucher, or the fact that we went from a single jointed snaffle to a french link, but she is clearly happier about it, with less negative reaction to bit pressure. So, I guess I’m not opposed to trying different things to see what works best for your horse. I really wish my main horse would tell me clearly what she prefers, but she actually seems to go the same no matter what I do.