Basically this its a matter of OC(Oppurtunity Cost). What you are losing in order to gain something else. In this case lets say u refresh FS with about 6 seconds left. You are gaining a refreshed FS in preparation for LvB but u are losing ~2 LB. In this case along with most other cases you can see tht the refreshing of FS early is a cumulative dps loss over a 2min+ fight.

