I have to say that I always felt the marines were a little more psychologically motivated than regular army personnel, regardless of what empire or nation we're discussing. That's just my impression.
Like you say, they're the first ones "on the beach" or in the drop/landing zone (mostly anyway), then the army brings its mass to the fight. The marines, given the US model, like to be a trim and mobile fighting force. To me that speaks that the army, though no less capable, may have some heavy equipment that the marines don't. I don't really know. Do the marines of any nation field mobile artillery? Maybe the Imperial Army has the much hated (and discussed) "mecha" division and/or corps attached to high tech worlds, in addition to regular grav-tank armor. I'm not sure. Just tossing out ideas here
GT may have dismissed the Imperial Army as an anachronism, or something that never existed, but I think GT needs to catch up and follow now only some of the arguments here, but the real practicalities of having a standing heavy fighting force.
I think it's either GT Ground Forces or GT Mercenary that states that planetary armed forces have a variety of organizations; from Impis to warbands to "armies" of only a few hundred heavily armed men and the like. I can't imagine such a hodge-podge selection of forces being asked to coordinate, spend time together in jump for a few weeks or months, and then deploy to some far off world for action. Particularly when you're facing oppisition that clear has regular standing forces, or large scale forces.
Example; the Aslan might be the closest thing that both GURPS military supplements refer to when they say there's no Imperial Army, in that the Aslan clan heirarchy has no supreme standing army that oversees all clans, and calls upon units from clans to serve at separate installations and bases as one cohesive fighting force. Instead the Aslan or more like what GURPS describes for Traveller; a kind of polis that can offer troops to the clan leadership when needed. And that said troops might be highly disparate in terms of training and equipment, as per GURPS description of its version of Imperial regulars. So, I can see the Aslan having that kind of structure.
But certainly not the Imperium's neighbors, especially not the Solomani (Terrans). The Hivers? Meh, maybe, I don't know enough about them. The K'Kree? They might have something close to the model, but again, largely because of their herd mentality it would seem unlikely. The Droyne? Heck, they provide troops to whoever. The Vargr? Yeah, they're more like the GURPS Imperial Model than anybody else, and then some. The Zhos? Probably not. Even though they're tech is a bit substandard, and they're snobbish because of their hoity-toity superiority complex, they might have some anachronistic individual regiments for their privileged classes, but it seems they would have a uniformed standing army. Darriens, Swordys, minor client states, pick your poison/race, it seems that if you want to keep your neighbors at bay, then you better mimic their model lest you try to defend a world with a mish-mash of TL-1 barbarians in vacc suits with spears trying to coordinate with Bwaps and Vegans and Geonee troops armed with a variety of different weapons and equipment.
In short, given the Imperium's size and modus operandi, I've no doubt that there are private armies for corporations, nobility, and other private parties, all using their personal standards for training and equipment, but the Imperium needs to maintain its territorial integrity, and needs a cohesive, functioning, fighting force other than the navy and marines to help it send a message of preponderance to its neighbors, but also to maintain internal order.
I think this is a mildly tapped subject that really hasn't been addressed properly.
More brainstorming; various regular units across the empire might have some varrying degrees of preparedness, but they all have to answer to the same command structure for the Imperium.
Forces other than Imperium, let's say Norris has some personal Dragoons that fly around in high performance grav attack craft. Certainly tey can be added to the rosters, but they're his troops, subject to his command, and perhaps not those of whatever high ranking army or marine officer is on board that transport/cruiser/transport that's headed to some world embroiled in a conflict.
Just my imagination working overtime here. Don't mind me.
