The usefulness of marines depends a lot on tactics. If you try to use them as shock troops, plunging head on into a border city defended by Mech Inf, tanks oreven Infantry, you are going to lose a lot of marines and battles.
On the other hand, if you utilize their unique ability, they can be very useful: Fill a transport with them and another with mech, inf, a worker and some tanks. Swoop in on a city AWAY from the border or main conflict. Often that city will be defended with spearmen or pikemen and the marines will triumph. (use a sub, explorer or spy to find out which).
Attack directly from the sea with the marines and hold the city with the other troops.
This tactic is *very* useful to capture a city strategically important because of geography, to secure or deny a resource, to establish a foothold or beachhead on a continent or to create a second front and cause the AI to respond to a threat away from your real objective.
They can lose some of their usefulness later when tanks etc are more common. But even, then the AI likes to 'parade' units like tanks around as opposed to fortifying them in a city for good. Often you can take the city even though there are tanks nearby (holding them is harder though).
You can also or subject the city to heavy bombardment first, then send in the marines; but the element of suprise is sort of lost that a squad of armour would work just as well with less support requirements (like air support) for the invasion.