Kyuuki's fine to keep as a sub eidolon in your wind team for the stats. Also, her summon effect being 15% thunder resist has its niche use.

Mastema's... rather useless, yea. Moreover, as of this post, thunder still doesn't have one of those eidolons that depend on the # of sub eidolons matching the element. So between now and whenever thunder does get one such eidolon, one can pick up Amaru, Tartarus, Medjed, Meng Huo, Ixion, and the thunder lilim to fill out the sub slots.

Mastema staff is a case of pick up a MLB copy just in case:
1. Mastema 2nd reprint occurs at some point on DMM
2. Game is still open a year after that for us to get it
3. You possibly want two FLB Mastema staves in your thunder grid at that far away point in time
All in all, Mastema staff is a low priority target. I'd probably work on it after I finish up Kyuuki bow (just need 1 more copy), but I'm also already done with Kyuuki the eidolon. And I'm skipping Mastema the eidolon.