Both Serah and Noel are tank-type Defenders (like Fang, Vanille, Sazh and Snow) rather than evade Defenders (like Lightning and Hope). They would not need Elude since it would somewhat contradict with their tank ability set. The agile ability set doesn't really function very well against magic, and I'm guessing that there's a lot of magic usage in this game on the enemies' behalf. Edit: It's possible that some SEN monster allies might have this ability. Not sure. Maybe someone who's played the game may know.
I do find it strange that they only have seven abilities each (at least according to
the wiki) and not much setting them apart, though. I would have expected Serah to share more of Lightning's more agile ability set, but instead, she shares Snow's. Noel is the one whom I had expected to share Snow's ability set. I think it's weird that one has Mediguard while one doesn't.
Some of the ability distributions in this game are kind of strange to me. In FFXIII, they kind of made sense for the character. For instance: Snow's the hero--he gets the best abilities in terms of being a Defender. Lightning's agile, so her Defender abilities play upon that strength. Hope is almost like Lightning's pupil, so he inherits her Defender abilities by default... and he has the "revenge"/counterattack abilities as well. Vanille is good as a support or mage character, like the yin to Fang's yang. And so on, and so forth...