The main reason for "NOT" using fire formed brass for head space is that the chamber it was fired may/is not the same as the chamber you are cutting.
I will always use the SAMMI head space gauge (GO) to set head space, but will check factory cartridges in it to make sure that they can be fired. But I will never exceed the go gauge by more than 1 or 2 thousandths.
Belted cases are not very consistent brand to brand these days and should be checked against the Go gauge.
The main advantage of using a SAMMI or custom gauge that is required for wildcats is that once you fire form, the dies can be adjusted to get the amount of head space you want/need and most factory ammo/brass will work.
J E CUSTOM