kraigwy, a lot of us have been known to do that.
Some trainers, these days, train to physically manipulate the slide instead, for the following reasons:
1) Fine motor skills (slide release) vs gross motor skills (racking the slide) - assumption that fine motor skills are more likely to degrade in a stressful situation;
2) Universality across pistols - some pistols don't lock back on an empty chamber, so by training students to rack the slide, the instructor prepares students to handle any semi-auto the students encounter;
3) Commonality of movement - the same motion used to routinely load the gun is used for stoppage clearing, so it's easier to instill muscle memory;
4) Control fit - IE, some people can't reach the slide release without substantially altering their grip on the weapon.
You may or may not agree with any of the above, but those are the primary reasons I've been given by those who teach "running the slide," as the OP phrased it.
Personally, I still go either way - IE I use my slide releases pretty often - but I taught my mother to rack the slide, in order to keep it simpler for her.
I also use both techniques, but VERY heavily favor the slide release as far as reloads are concerned. To date, the slide release has failed me exactly zero times, and I have never immediately reached for the slide release in case of a malfunction... I always go straight to tap/ rack, as I have always trained.
I completely realize the benefits and commonality of failure manipulations associated with the overhand technique, and I don't discourage it a bit. But what I can't stand is the "overhand only" people that cry blasphemy if you use the slide release. Our range master at my agency is one of those people, and he is why I get so wound up about this topic from time to time, so please don't think that I'm putting anyone in this thread into that category.
I heard the best argument ever as far as the fine motor skill myth the other day though in a class with Frank Proctor (wayofthegun.us). He said pointed out that the slide lock/ release is roughly the same size as the magazine release, and requires much less "fine" skill to operate than to effectively operate the trigger. So if those actions can be performed, so can manipulation of the slide release.
I've also never understood how most (LE) trainers preach muscle memory on so much of fundamental work, but somehow we can't apply muscle memory to the slide release for some reason.
When training to the lowest common denominator, "running the slide" may indeed me the best bet. When training shooters though, slide release is where it is at (IMHO