Keep every gacha SSR eido that you get, no matter how painful it is.
You'll eventually get to MLB them in forever, after which you get to use their superior stats compared to event eidos (well, some of them) and perhaps use a few actives without worrying about stat penalty (I love Ouroboros's active so far).
Can't remember which gacha eidos are so trash they don't even deserve a chance at MLB off the top of my head... but maybe stuff like Fenrir can meet their fate in the incinerator, at least IMO.