In our games, no one plays a priest, commisar or Engineseers. why? We all know our GM prefers it that way, basically. He likes the idea of priests and "political officers" being NPC's that he can use to direct us and put them above the guardsmen. If PC's played them, there would be the danger of comraderie, and well we just can't have that in the IG.
Anyway, we're all actually okay with this, it gives us Guardsman classes someone to be wary of even in down time and sets a precedent that there are people looming over us that are outside of our own command structure.
It also makes sense to me on many levels. The above mentioned classes don't seem to have the same flexibility as the guardsman classes do.
How do you guys run it?