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.