I think that Rei is just confused, because there is no reason to not agree. OP is not saying "please remove NIN running animations and MCH heated animations because i said so", he is just asking for a toggle for own character. It hurts no one, there are no downsides to this. So if you just come and say "I don't agree" without arguments, it brings nothing of value to the discussion.
The only real question that can be asked is it worth devs' effort. I believe it is worth it for two reasons. Both NIN running and MCH heated animations deeply change how the jobs work.
NIN naruto running makes it 100% "weeb" job, and i know we have a lot of rogue fans here, so option to toggle it off would make the job more appealing for them, since i doubt there are any chances for Rogue job.
MCH heated animation change the job from Gunner to acrobacy themed job. Meaty gunshots are removed in favor of "blower" sounds. These 3 animations ruined whole job for me, and it appears many more people, 3 years ago. Also if you look at this poll
https://forum.square-enix.com/ffxiv/...%28Post-ShB%29 it appears there is high demand for Gunner job. With that change MCH will finally properly use a gun, so i see no reason to add Gunner job then.
Devs' already have some sort of glamour system with SMN egi glamour, and /bahamutsize and /gbarrel commands, so i'm sure that developing server side toggle is a matter of hours.