Whereas, one would not choose Marines over army if a long campaign was envisioned...
Whereas, one would not choose to have Marines doing pacification work and long-term planet or polity building.....
Marines are and will continue to be the fastest response mechanism available to the Imperium - travelling as fast as communications (that is to say, Marines Always Ride In Navy Equipment (MARINE)). So, they'll be put anywhere the Imperium deems it necessary to deploy force on short notice - it may be show the flag & intimidate the locals, it may be bash the crap out of some pirates or Vargr (as if you could distinguish...), it may be drop on a penny-ante local potentate with delusions of his own larger importance about to be suddenly corrected, or it may be as the spearpoint of a large scale planetary invasion during wartime.
They travel fast, hit hard [1], and they take and hold ground. OTOH, they probably (like modern MEUs) have SF and advanced recce capabilities as well as all sorts of integral support arms.
What they are not is a long-term substitute for the Army and COACC and Nautical Force guys. They can do the job 'as a stopgap'. But using them for long term ops like that is either wasting their capabilities and taking them out of peak fighting trim (doing OOTW on a planet isn't good training for opposed boardings on starships!) or it is tying up a useful mobile unit in a static role. Or both. But, if needs must, s'be't. For a time.
They are the 'hole fillers' and 'fire brigade' of the Imperium. The Army is more potent (in that it is much larger and can thus bring to bear a whack of assets), but they deploy more slowly and aren't as mobile.
Or so I say
[1] Marine forces can be deployed down to platoon sized forces for recce purposes, operating off of destroyers or the like. Any Marine force will have high tech firepower out of proportion to its TO&E numbers. But it will be a small TO&E, all told. These are the Few, and the Proud. But when it comes to large scale long term conflict, Proud isn't all that useful and Few is a key issue. The Army may be a buncha Dog-Soldiers (not Vargr, like..), but there are lots of them. Just what you need for pacifications and long term OOTW. Boots on the sand/dirt/frozen hydrogen/permacrete/whatever.