Until recently, elevated frame mounts were the only game in town as the optics were not capable of withstanding the g-forces encountered when mount to the slide, and the optics were much heavier and would have negatively affected the cycling. With the sight elevated this high above the bore axis, at any distance other than the sweet spot at which it was sighted in, POI is way off from POA. Also, you have the problem of slide-to-frame alignment; as an example, the loose fit of a Glock slide to it's frame results in considerable inconsistency of slide/frame alignment from shot to shot, and between loaded and unloaded chambers -- to the tune of inches at only 7yds.
If you are talking about a big scope, you may have no choice, but if you are talking about one of the new, rugged miniature RDS, you will have a much more functional/accurate weapon if you mount to the slide (preferably milling the slide). Have you looked at the WarriorTalk forums at http://www.warriortalk.com/forumdisp...ighted-Pistols