I did some DPS comparisons and the Lilith eidolons are a substantial DPS loss unless you actually plan to run with 2 off-element kamihime. As in, a team with 1 off-element kamihime and typical eidolons will outperform that team in DPS by 5-10%. It's a nice fallback for Ragnaroks where certain skills are required, so you definitely want to pick up the one for your element, if only for their situational usefulness (an ~15% DPS boost in those 2 off-element kamihime scenarios).