Very useful for getting good groups. When I was doing SR, I was shocked how many people didn't use slings (and their scores reflected it). The trouble is, when using a sling in prone position, you have to be aware in the changes in impact it can have when you don't use a sling (OR, in other positions where sling tension varies).
This is most pronounced with non-free floated barrels (barrel flex with sling tension).
However, one can avoid alot of that by "shooting off the magazine" (in the case of AR type rifles in prone position), and not tensioning the sling much, if at all.
I still like to use slings in sitting and standing--it just helps with being steady.