His vs "His or hers"
I can see only three plausible reasons for the practice. One is that the fact that the person in question might be either male or female is important to what you are saying, and so should be emphasized. Thus in one passage, where the point I was making was that you wouldn't know anything about the person--the context was encrypted interaction online--the editor changed "he" to "he or she" and I changed that to "he, she or it." But in most of the text, gender is simply irrelevant--the points I am making would be equally true in a world of all men, all women, or all hermaphrodites.
The second possible reason is that "he or she" is seen as politically correct, thus using it instead of following the traditional practice of letting "he" stand for either male or gender neutral shows the enlightenment of the author. This strikes me as at best silly, at worse conformism at the cost of good writing.
The third reason is that the writer wants to make a political point--that the traditional practice reflects features of the society that writer wishes to criticize. This seems to me a legitimate reason if that happens to be a point that that writer wants to make. But it isn't a point I want to make, and I strongly object to being drafted into someone else's crusade.
That said, I will happily agree that the lack of gender neutral pronouns in English is a problem. With the one exception mentioned above, I have accepted (so far) none of my editor's alterations from "he" to "he or she." To my ear that is not only clumsy--three words instead of one--it is misleading, since it suggests, almost always incorrectly, that there is some particular reason for pointing out that the person in question might be female. What I have done is to look for another way of writing the sentence that works at least as well and avoids raising the question of whether I am talking only about males. Sometimes that is possible, sometimes it is not.
Another possible solution would be to give in to the common practice of using "they" as a gender neutral singular, a practice I abhor. I suspect my editor does too, since she is enough of a grammatical purist to insist on treating "data" as a grammatical plural and to distinguish carefully between "who" and "whom."