I don't know. If I had to guess...

I use Nagi but she's isn't something I dedicate a spot of my team for.

I cycle her in when I think additional damage would be nice against a boss or a stall war.

Ninjas have around only 300 to 400 attack outside of skill and they're more for masses of low defense enemies.

If you're not dropping ninjas to "win more" than you might be better off using higher attack archers or a good blocker. Mages can also fulfill the role later in the map.