Good point. One of my favorite aspects of 4X games is eXploration. Certain Civ IV map scripts can generate "new world" continents and islands. The first civ to explore and settle them can become extremely powerful from the additional cities and overseas trade routes.
I'm mostly familiar with Master of Magic and not Civilisation games, but if the same is true for all of these games it indicates a deeper problem. Apparently eXploration is only an early game thing ! This is not very fun, and not very realistic either. In real life, exploration (discovery) of other continents required advances in navigation, cartography, communication, transport... I think requirements for eploring further continents are not harsh enough in MOM. You can scout the entire plane with just your starting spell Magic Spirit. Fortunately there's still Myrror plane, and you need either a spell or a strong military force to break in. There could be more things like Myrror, and with different requirements. Myrror was mostly about having a strong enough army to punch through.
I think in this case limiting long-range exploration units would be beneficial to gameplay. MOM gave too easy access to fast, floating Magic Spirit. The point would be not to make eXploration strictly worse, but rather introduce more stages of exploration. You'd need better ships or other workaround to reach farther continents, for example.