If your camera delivers as expected on the assumed promise that it will always pick the lowest ISO given the other settings you've selected, more especially if you are shooting moving subjects, there's very seldom much reason, short of shooting for a special effects shot, to not use auto ISO because in good light it will pick the best/lowest ISO just as you would in conjunction with your other settings and in bad light it will pick what it needs to way before you can in order to let you walk away with shots you wouldn't have gotten otherwise even if it requires that you apply a little noise reduction/removal software. When shooting still shots where you only want base iso used, and may even be shooting in "bulb," auto ISO certainly could serve up some undesirable results.

If your camera delivers as expected on the assumed promise that it will always pick the lowest ISO given the other settings you've selected, more especially if you are shooting moving subjects, there's very seldom much reason, short of shooting for a special effects shot, to not use auto ISO because in good light it will pick the best/lowest ISO just as you would in conjunction with your other settings and in bad light it will pick what it needs to way before you can in order to let you walk away with shots you wouldn't have gotten otherwise even if it requires that you apply a little noise reduction/removal software. When shooting still shots where you only want base iso used, and may even be shooting in "bulb," auto ISO certainly could serve up some undesirable results.

There's six kinds of people that I can think of offhand who don't/won't use auto ISO:

1) those who don't know about it,
2) those who don't understand how it works and its benefits,
3) those who never shoot anything but still subjects and mostly in good light,
4) those who like to shoot in early and late daylight but either shoot timed exposures and otherwise
won't start shooting 'til they can apply some variation of "Sunny 16,"
5) those who like to miss a lot of action shots due to the inability to rapidly adjust ISO on demand in
insufficient light fiddling with a special function button or users whose ISO setting is inconveniently
buried in a complex menu,
6) those who like to start shooting much later than others in the morning or have to put their camera
away well ahead of others in the evening because the light isn't "perfect," just as it used to be when
that person had 50 or 100asa film loaded and couldn't get a decent shot due to insufficient light -
those who've "by god, been doing it that way for fifty years" and ain't about to change regardless of
innovations that come along that allows them to overcome those deficiencies of fifty years ago -
those who, "if Ansel wouldn't do it, by god, neither will I."

Admittedly, some have higher standards than others, especially those who do it for "hire" who have to please customers. Those folks get special dispensation from my list above. By the way, I was around about twenty years before Ansel moved on and I don't recall there being anywhere near the fuss that's made about him now. Seems like he was just another goober running around with a big camera.