This is true, but there is a cap on how many of each type of unit you can have at one time. For example, if you have two Planar Gates in cities that each have a Mage Guild, then you can have up to 2 Mobius Witches. Once you have two, you don't get any more...whether you have access to other Planar Gate units or not. If those two cities also had Carnivals in them, then you would be able to get up to 2 Chaos Marauders. This would not affect the maximum number of Mobius Witches you can have. Choosing not to build a useful or necessary building so that you won't be burdened by free troops doesn't make a lot of sense.