How to use mage in a battle

[deccan]

Tuidjy said:
I am always amazed by some people who keep giving advice that presumes that
their opponent is brain dead. The reason you will not see armies of 50 mages
except in artificial winner-takes-all battles, is that big armies cost a lot,
require huge upkeep, move glacially slow, and are decimated as soon as they show
up outside a dome.

So does this mean that mage-based armies designed for using communion generally don't work in practice?