As the XD has no thumb safety, it is necessary to push on the back of the slide with the thumb while holstering to prevent the gun from going out of battery (at least with a tight holster).
Holstering in this fashion happens to also prevent engaging the grip safety.
Being a 1911 and revolver carrier primarily, I forgot about this. But it is a good point. I also think it is a serious disadvantage and I would not want to carry such a combination that required this.