This spell is almost a requirement for low to mid level co-op (under LVL 80 or so), as it boasts very low FTH requirements (just 12 FTH!), is easy to get, and provides a ton of HP per FP per cast. This lets you conserve HP flasks while running around in the overworld, where the slow heal rate doesn't bother you much because (1) you have less health overall (2) enemies don't hit as hard and (3) there is a lot of down time in between your taking hits (hopefully). Even if you have access to the FTH boons (which require 24 FTH or more), Bestial Vitality is still worth running, as it is vastly more FP efficient then the Boon spells.
It gets less useful as enemies hit harder and HP bars grow, but is still useful / worth slotting even up to 60 VIG.