I disagree that marines are useless, but they and paratroopers ARE specialty units.
If you use battle tactics besides simply massing units on a shared border they can be invaluable. I will often start a war on the border *partly* as a diversion (maybe the secondary objectives are there), then allow a turn or two for their troops to respond. Then attack a coastal city (near a needed resource or to establish a foothold)
with a marine company. Obviously if defended with Mech Inf or even Infantry, it can be dicey; but if defended with Musketmen or less, the city can easily fall, and now you have a foothold way behind their lines or on a new continent.
Follow that invasion up with a transport of Tanks, Infantry for defense and maybe some workers.
Early on, I would use an invasion force of 5 marines, 1 or 2 Tanks/MA and 1 or 2 Inf/MI in a transport depending on the defense (a sub or Explorer helps decide).
Lately though, I load up 1 or 2 transports with marines and 1 or 2 with armour and maybe one more with workers and settlers and cruise up and down the coast cherry picking cities.
Paratroops are a bit less useful (they dont do anything Infantry in a copter couldnt - barring editor mods), BUT you can drop 6-8 of them over enemy lines as a feint, let the enemy respond then launch the real assault elsewhere. Otherwise, you sort of have to be lucky to have an airport close enough to drop them into a fight, near a target or into a city as reinforcements.