So Darth Maul and Grievous are obscure, then?
Aayla isn't as involved in the story as those two. While still a form of eye candy, they were main villains with which the Jedi fought and integral in the story.
Aayla just added hotness to the Arena battle on Geonosis and came back in ROTS to have her hotness in again for a few moments. Her shots in both films could be taken out and the movies would not be affected, Grievous and Maul would leave significant gaps.