[Quitti]

While the posts above sum it up pretty well, I'll point out the obvious age difference those mages usually have to your hard hitters, which usually (with exception of some nations) are old age. If there are spells up such as Burden of Time, it kills your old age mages fast, and while those initiates and such won't probably stay alive dozens of turns either, at least they outlast pretty much every other mage you have.

It also pays off to research spells specifically for those small researcher mages to shoot out in the battlefield, eg. communion slave&master + soul slay for MA C'tis and their lizard shamen, which are their best gold per research mages. Same applies for seithkona (utgård), but you'll want nether darts, and so on.