I believe that one of the main arguments behind the refractive limit is the increased risk of retinal detachment. This risk is, according to the JAA medical manual, greatly increased at around -6 diopters. In the mentioned manual you can also find a chapter outlining the level of acceptable risk regarding incapacitation. These combined points, I believe, are the arguments for maintaining a refractive error limit.
Having said that, I do have doubts as to the level of legal certainty involved in having such limits set. My doubts have arisen form the differing ways in which one's objective refractive error (shape of one's eye) is determined, and from comments made by an expert in the field of aviation ophthalmology.
I've heard comments indicating that some applicants have been tested in one way, whereas others in another (subjective vs. objective methods of determining refractive error). This coupled with comments from experts stating that the limits are politically set, and the fact that anyone can be unfortunate enough to suffer from retinal detachment, have left me wondering.
I have no objections to setting standards for our profession, but they must be such that legal certainty prevails. Personally, I believe that the UK CAA is on the right track in pursuing the abolishment of refractive error limits (this according to what has been posted on this forum previously).