I guess I just don't understand why event SSR eidos quickly turn bad, if the eidolon effect is irrelevant when it's not main. Is it because the summon attack is bad?

I was under the impression that if I manage to farm enough mats to break limit this SSR eido 3-4 times, it will give more HP and attack than any BL SR eidos I find in the future, and thus be good in the long run too as one of the 5 sub eidolons. No?