Her stats make a lot of units pale in comparison. Sure, Gellius has great defense, but Nadia has defensive stats AND offensive stats. I know you love your argument of "rather have a tank and then place out mages." But think about this - with Nadia out you have one great damage dealer and a tank out at the same time. Sure, she costs more, but she's like one hell of a monster that effectively manages two roles at once.

Gellius on the other hand is just that - a tank. He is only useful for blocking. Nadia can smash things to pieces as they come towards her. Who needs to deploy that archer or mage to go with Gellius? Just put down Nadia and be done with it.

There's also many maps where ranged slots are rather limited. The current subjugation event on DMM only has 3 ranged slots. Nadia was born for that map...