Ideally I would prefer someone in a Ministerial position to have some grounding in that area. One aspect that an ex-military background brings which has to be a positive in any S of S or Minister for that matter, is that the military are taught Leadership.
Frankly, leadership and the realisation that they are responsible for their actions are both sadly lacking in a great number of our Ministers.
Not that I am saying that all MPs should be ex military but at least they have been leaders in their professional lives. Not sure if a PPE, History or Economics degree provides the same grounding.