For me (and not only for me) it has to do with the sensitivity, when it's really low you can do it with the wrist with the same accuracy, at least I can. I never felt the need to use the fingers with 55+ cm/360
If we talk about few pixels movements, sure you can help yourself with the fingers in some situations, but that's it, and depends by the grip as well.
If you palm or claw you have a very little space to move the fingers and if the sensitivity is really low, often you won't have enough space for significant microadjustments and you just destabilize your aim if you force the use of your fingers or if you stretch them too much trying to microadjust. That's my experience at least.
Instead when i switched to a higher sens after playing faster games, my grip naturally changed after switching mice and I felt the need to microadjust with fingers cause it was actually really effective and it wasn't hindering my aim in any scenario .
This image shows one of the best CS players not doing microadjustments with fingers like you say. And there are many others.
Remember we all react differently so it's hard to say if something is right or wrong. For me, the game I play, the mice I use and especially the sensitivity, are the factors that determine how should i move the mice.