While it is true that IVs matter the least out of the whole package, they are also the only ones that can be selected by the trainer, while at the same time can’t be changed on a caught pokemon.
Base stats are the same for the each specimen of a species. You can’t change them.
Typings are part of the species and can’t be changed
Movesets can be changed via TM, unless they are legacy.
Levels can be increased by powering up.
That leaves one only one factor left that is set in stone for on individual specimen of a species, which are IVs.
The main reason why people pick on IVs is because they can’t be changed and because powering up and maxing a single pokemon takes a lot of stardust and candies. So why not get the best for that, instead of wasting it on something that’s middle of the road and that you might get a better version of in the future?
I do short man raids from time to time and in general I prefer to have a team that can show its full potential. Due to that I have a personal rule, where I only power up pokemon whose IVs are above 90% and have at least 14 attack, since in raids said stat is way more important.
As of Mewtwo mentioned above. I personally wasn’t able to get a psystrike during the week it was here, due to not having time in the first days and later getting a suspension due to Niantic’s unintentionall Xiaomi ban, which was resolved after Mewtwo was already gone. In the make up raid hour today I got 5. (14/13/11; 15/12/12; 11/15/14; 14/11/15;10/12/10).
All are way worse than what I would normally power up, which is why I will see if I’ll get lucky with some lucky trades. Hopefully I won’t get screwed over with garbage 12 and 13 attack luckies. Otherwise I’ll power up either the 14/11/15 or the 15/12/12. At the end of the day, Mewtwo won’t be that useful due to psychic in general not being useful outside of Machamp raids and the upcoming Terrakion raids. So having one maxed out is enough for me.