So I guess, in a nutshell-- for this party, I came for the animals, but I stayed for the aliens
Ditto. As a kid, if the covers didn't show kids turning into animals on the cover (or just animals in general), and if the covers had pictures of andalites and yeerks and yeerk pools and spaceships etc... I don't think I'd have ever picked any of the books up, let alone read them.
However, now that I'm not a kid - yeah, the kids-vs-aliens intergalactic-space-war theme would attract me to the books and I would probably read them even if the ani's didn't turn into animals (comparable to something like the Tomorrow series, I suppose)... althought the series would then lose its "uniqueness" in my mind
The animals caught my interest, but the aliens won me over