Because these units would be more expensive than normal units, you'd have to balance the extra maintenance costs vs. the benefit of the unit being stationed in the city. This leads to bean counting, so you'd have to be very careful when implementing stuff like this. These units would become kind of "mobile buildings," unless the bonus was small enough to only be used circumstantially (like when the unit is passing through a town), but then what's the point? I think these kinds of bonuses are best reserved for heroes.  

But some buildings can't be stacked, so to speak. Units can be. You can build 100 "love charm" soldiers that do nothing but boost population and stick them in each newfound city you create to get it rolling. The whole mechanism kind of abstracts the game and makes it less fun (my perspective, of course) and less logical. 

Like for example in Civilization you can't just build twenty theatres in a city just to get some huge culture bonuses flowing. 

Yeah, maybe you could have the same bonus not stack at all - either a city has a "love charm" bonus or it doesn't.  One can only do so much "loving", after all.  Or you could just have it stack up to X times, or diminishing returns.

Obviously it would have to be carefully balanced - but every aspect of a game needs careful balance if you want it to stay interesting and replayable for any period of time.  That's no reason not to implement things.  Even if it got cut from the final release, if the framework was there, people who were so inclined could mod it in with minimal effort.