yeah, that's pretty bad unless you don't have any other eidolons to use.
the best things to put as subs are eidolons with high stats, or eidolons with good effects on call.. even better if they can do both. it can even depend on the event as well. (like using a Gale Kaiser Dragoon to block some thunder damage against a thunder enemy).
for instance, here's the eidolons for my wind team:
Attachment 11677
My Main one increases Wind elemental atk by 40% and reduces thunder damage by 10%, so she's placed in that slot.
As for subs:
-Echidna (dark) cause she has lv100 and high stats being a gacha one (her dizzy sometimes lands too).
-Meng Huo (thunder) is there for both her lv100 stats and her atk buff when summoned.
-Huanglong (thunder) is there for stats like echidna. She rarely gets summoned though as she just extends the length of stun mode on bosses.
-Tiamat (Water) is also mostly there for stats, but her summon effect of adding a 3 turn barrier against damage can help soak it up (usually pops instantly cause it's only 750 HP damage it blocks total).
-Gale Kaiser Dragoon (Wind) is there for her effects more than her stats. Her summon effect provides a very good buff combo of boosting wind damage I deal while lowering thunder damage I take for 2 turns.
it's honestly not just the main eidolon, but also sub eidolons that are important, as some can greatly help you (like Meng Huo and Gale Kaiser's buffs greatly increasing burst damage in a full burst combo). At the very least, they give more Atk and HP and that gets amplified when weapon effects and other things come into play.