You use it to keep the twin snakes buff up.

Yes, you can formshift when you have to disengage, but also if you have to disengage and stack or something like that, you might as well use it. It's the difference of hitting a move to gain the damage up buff while it's not in effect, and hitting the skill with the damage up already in effect. It's minor, but it is still more damage.

I use it, as an example, in Matoya's Relict in the second boss fight while the party is on the cloud after killing the adds and waiting for the attack to go off (after doing all my meditates)