well, it helps to know what you're talking about
Personal experience is one of the best sources of knowledge, of course; in some respects -- indispensable. But even without such, one's opinion on an issue -- any issue, actually, not only war -- could still be valuable if one studied it first.
John Keegan has never been to war, but then he's a professional war historian, so his books are good and deserving (though he's not an undisputed authority: if you read military mags you'll see that he's been actually criticised quite a bit.) But at least he's a respectable author, you wouldn't simply dismiss him out of hand. Thucydides was a general, so his book is very valuable. Same for Caesar, or Patton. Remark's books are good -- but then he'd actually been as soldier.
Otherwise... well, of course a person can have an opinion on any matters whatsoever -- you can't forbid one to have opinions -- but what's the value of it? You wouldn't hire a plumber to teach you physics, you see... though he may have opinions of course.