actually , u will use odette a whole lot more then karma and anya to be honest. They are very strong units , but they are a whole lot more niche. Odette and belinda are very good event units for the simple fact that they are very general use , and there aren't that many other viable options for free 2 play players
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They are more reliable in the fact that they are used a whole lot more then mages AND you can't really replace the witche's utility with any other unit (well , maybe mikoto , but that's it) and also u will NEED to use witches (phalanx 2 , war of magic , etc) for certain maps.
On a side note , mages are really expensive , and a whole lot maps on which they would be usefull , they can't be placed because of their cost. Besides that , they can be replaced with stuff like summoners , or even cannoners , so they are not always a "must have" unit in a party