Stationing troops in that city to add the bonus is not something the AI will likely understand while a human will do it aggressively if its available. Gaining small amounts of XP while in a city is one thing, gaining an otherwise unobtainable bonus is another.
I don't find that units from other cities are effectively crippled by not having this bonus, it's more that the ones that have it are the elite unit core where I am encouraged to build most of my offensive and strategic units there and use any other cities for support units, garrisons, and additional mainline forces. My Tanks or Swords or Siege units always come out of the Hero Epic city. My Infantry or Bows don't. Ditto Battleships versus Destroyers.