Of course it's all in one's taste as to what figures one would want to use as for their Imperial Army. Yes the Imperial Marines are great for doing ship to ship fighting and the initial ground assault or to quickly crush a revolt of some miners on some forgotten asteroid but for your longer ground campaigns or for an outpost in the middle of nowhere your going to need an Army.
Now go out and have your players look for work in a bar half full of Imperial Marines and the other half with Imperial Army with nothing but time on their hands and their brains half polluted, toss in some locals and remember everyone is carrying a loaded weapon! Oh yes it's going to be a good time at "O-Mally's" tonight! Then in time someone is going to call the local police.