It's fine to collect them but don't waste your time leveling or farming them. Unless they have a very special buff not found from past/soon to be seen event eidolons, they're garbage. They're barely as strong as a R eidolons who take less effort to reach better stats.
I grab every copy of advent event weapons for collections, meaning all the SR weapons along with the SSR stuff, but that doesn't mean I keep them or use them. Having own them once is enough to fill their slot in the library and anything SR HP skill related gets used as skill enhancement.