Units can improve, stay the same, be weakened or be destroyed in any battle they participate in.  The more trained they are, the more likely they are to survive.  Low discipline is a good thing, yes.  Also, bear in mind that some ships (Vipers included) are for military only.<br/><br/>EDIT:  mia h caught it already.  Also, rocks are also for military.  Civilians can have dirt clods.<br/><br/>Using a garrison as a police force has pros and cons.  It is better than nothing, but not as good as a security force.