I totally agree, but the shrills were part of the army, too.  And if that were the case, the army could be split and the air elemental could have moved forward alone, and that would be a cool consequence of the spell as well...  

I just wish they had established all of the movement parameters of the units prior to building the game.  I am pretty sure flying was introduced after FE was in beta for a while and a lot of potentially great units (griffins, giant eagles, scouting falcons, etc.)  were never created.  They also jiggered with mountain and mountainside tiles during the betas as well.