Military experience is beneficial but not a requirement to be a historian. Some of the leading military historians never served a day in their lives.
David Bercuson comes to mind. Stephen Ambrose had limited military experience also. Still, I would place Granatstein above either, and he did serve, briefly, as an officer in peace time.
Roy Farran was a soldier's soldier who served with the SAS and had more medals than most people have pairs of underwear, but his regimental history of the Calgary Highlanders was sub-par (though possibly not considered that at the time it was written - regimental histories have come a long way from 1945 to the present).
Donald Graves is an excellent historian who has never been in a war (AFAIK), but Brian Reid is in the same league - at the top of the strata - and had extensive experience as a staff officer (and it comes through in his writing).
As Britney points out, other fields are the same. The best book written on the Apollo program was by a non-astronaut, Andrew Chaikin. But how do you define "best"? As a technical brief, it sucked - as readable prose, it wins out.
Often, criticism of history comes down to how good a story teller the guy is. Graves tells great stories about the SALH, but also had technical knowledge that assists him. On the other hand, those with technical knowledge often write poorly for non-soldier readers who can't penetrate the jargon they use or the stuff they take for granted (see the early regimental histories like Farran's). If you read some early post-war regimental histories written by guys who were actually in the war, you had better know what a TEWT, FUP, LOB and Sausage Machine are, as those books were liberally laced with such terms and usually without benefit of a glossary. Often these early histories were intended as souvenirs for fellow soldiers and not with future generations or civilians in mind.
I don't know that university imparts any special skills on a military writer, either; the ideal situation would be a person with military experience, technical understanding of not just his trade but many, technical skill as a writer, world experience and education, and storytelling ability. I don't know of many people who fit all those bills. Lew Mackenzie comes to mind, Farley Mowat is a much better example. But there are excellent authors who don't have all those prerequisites, and they outnumber, I think, those who do.
Respect has nothing to do with it. Man is curious by nature. Any soldier who would be offended by someone showing an interest in his profession, no matter who it was from, would be shortsighted.