Just like it says on the tin. I'm in the middle of a game where Tarth is sending 2-3 armies at me every few turns, and they are almost always stacked on the same tile. The impact here is that you can't see (easily, if at all) everything that you are dealing with. Neither can you choose which army in the stack to attack. I've spent several turns working through these army stacks every time they happen, and if the AI were better at realizing the advantage, would make it WAY harder to protect my cities.

Ultimately, although its a bit of a crapshoot, this means that you could stack an army of 6 tiny militia units with a giant army of death, and you have an (apparent) 50% chance for anything that attacks your stack to blow up your decoy army and let your giant pile of military death coast on through to their destination (or at least have a better shot at it).

My preference here would be for one and only one army to be able to occupy a square at a given time. Alternately, we need a means of selecting which force we engage.

As an aside, I think this same "feature" has also created situations in which champions who were killed in battle for a city respawn as an army over the city tiles, and appeared unable to move.