Sure, we already have a lot of swords, but we only have two sword fliers. Elincia with Amiti coming in with a strong quad build would give Flier Emblem more options. Assuming Elincia is good, Blarblade Summer Corrin + Elincia + Emerald Axe Camilla would form an extremely solid core, with Summer Corrin being nearly Colorblind to begin with (which means a staff attacker wouldn't be THAT meaningful anyway), Elincia covering most of the things Corrin can't kill and then having some overlap, and Camilla to check Reinfart and co.
It also has higher Might than usual. 9x2 is 18 Might, when other legendary tomes just have 14. So they either take away the Spd penalty on Amiti, or give us a 10 Might Brave Sword...