What difference does it make if the Imperium does have an organized Army?
Because militia and short-term drafted forces tend to be much worse as combat troops than a professional military.
Some additional thoughts:
I could see the Imperium having a small professional army that is used in critical areas and is under the direct command of the Emperor and his immediate subordiantes with a much larger army that is raised by warrant locally when needed. The third tier would be local forces raised for the express purpose of defending a planet or system.
I would think these would be something like the French, British or US Army in structure. That is, there are a small number of professional units like say, an Imperial Guard, several large units of troops that are standing units of "line" quality and lots of smaller support units that require technical expertise like artillery, engineers, and such.
I could even envision a "Foreign Legion" of some sort that allows non-Imperials to join that is part of the standing military. These would be a cross between mercenaries and volunteers and because they are "foreigners" they would be seen as expendable.
The next tier would be the equivalent of Territorials or National Guard. These are local units that can be called up and are capable of serving away from home. A drafted equivalent would be possible but not all draftees would be available for off-world service. The Canadian military is this way as is the French.
The local military is raised for internal purposes only.
The professional Imperial Army would be TL 14 / 15+. It would have the latest equipment and be well trained in its use.
The Territorial / draft Army would be TL 10 to 15 depending on where it is raised and what was available to equip it when it was raised. Its TL would be moved up if the Imperial Army decided it needed to be by re-equipping and training it. Training would vary but in wartime these units would be given refresher training before being committed to combat.
The local military would be roughly the TL of the planet / system it served on / in. Training would be highly variable.
The professional Imperial Army would have professional officers and Nco's serving in it while the Territorial / Draft army would vary significantly. I would think given the presence of nobility and a class / caste system in society that many units would have political appointees based on who they were rather than on competence. Some egalitarian worlds might appoint officers and nco's by voting as an alternative.
The result would be that these units would have a very spotty performance in battle at first until issues of competence and training were sorted out. That would make playing such units interesting.
The local militaries would vary in this respect depending on their usage. On worlds where the military was respected and gave social mobility I could see it having very competent personnel in it whatever the TL might be. On other worlds with little internal or external threat the military would languish as an ill-equipped poorly manned force filled with the desperate and dregs of society.
Some of the professional units might be very prestigeous to be in. I could easily see several Imperial Guard units that take only the cream of the crop in recruits. There might be other units like special forces and such that are very demanding and hard to get recruited into as well.