It doesn't have to be a forced mechanic, but rather a choice (in the beginning), telling them that it might be easier to do this for a while. If it were enabled for all players... movement rates would go through the roof! Might that not actually make it interesting for some less active players? Not logging in but still moving with the army?
I cannot imagine a single marshal would not set his orders to "Automatically follow your army, always. You'll get fined if you don't."
