.. - why it is not good?

I've met this post in the closed thread, so I cannot answer it directly.
However I feel that the approach 'I'm a tank so I'm standing still inside trash mobs AOE, now go gather all your tools and heal me!' is kinda live and popular.
There are variations in the thread like 'I move out of AOE only if they apply debuff'.

https://forum.square-enix.com/ffxiv/...=1#post5833028

So I've registered just to answer on that post.

My general mental-checklist when tanking trash pulls is:
1. Can I move out of the orange without significantly shifting the mobs? Yes? Then shift the minimum needed (and ideally shift back afterwards).
2. Are the mobs probably going to die in the next 5-7 seconds, and I can survive this damage? Yes? Then just mitigate as best I can and eat the damage; the mobs will be finished off shortly and there's enough time to heal on the way to the next pull, and moving enough that the mobs might move out of DPS AoEs is counter-productive.
3. Does the healer seem to be really struggling for whatever reason? (Poor resource management, having had to spend all healing resources on a DPS standing in bad and eating damage like they're at a county fair pie-eating contest, lack of understanding of their healing kit, etc.) Yes? Then make it easier on them and shift out of the orange, even if it means shifting the mobs around.
4. Conversely, is the healer really good with their kit and has me as a tank feeling functionally immortal? Yes? Pop a second cooldown if applicable and hold my ground while laughing maniacally to myself. (That last bit may only apply when playing WAR or DRK.)
5. If none of the others are true, can I move a tiny bit so I only eat one orange blob instead of three overlapping ones or whatever? Yes? Then shift to take the minimum number of hits.
6. Failing all else, just move entirely out of the bad.

It's generally not so much a conscious choice to run down the checklist, that's just sort of a summary of what I know goes into the gut feeling of "should I stay or should I go now" when there's a marker proclaiming that the Bad Stuff goes right here under me in a trash pull. So far, this has generally served me well.

(Boss fights are obviously different, because generally in boss fights you're meant to move out of all the Bad Stuff.)
The quote above is about a light party (cause 'the healer'). Lets ignore low level dungeons and say that every player have its AOE skill.

(divided into multiple posts due to restrictions)