Tom - I agree with the spirit of your statement. But when looking at risk mitigation, there are differences in standards that could be relevant. From a risk mitigation point of view - you can't have CA's who are unfamiliar with the agency standards participating as CA's.
From my limited experience with NAUI CA's I'll offer this example. We had a NAUI DM who was crossing over by doing a PADI IDC. The CD asked him to shadow me, and I had him do a simulated CESA briefing for another CA acting as a student, while under my direct supervision. He instructed the diver to spit out his regulator at the beginning of the CESA swim. I immediately corrected him and reminded the diver that under PADI's standards - we must retain the second stage in our mouth. He argued that under NAUI's standards - the second stage is supposed to be out of the mouth. I reminded him of PADI's standards (in the Guide to Teaching regarding retaining the second stage during the CESA) and offered two reasons why this is a prudent practice (e.g. prevent the inhalation of water, and possibly getting additional air during the ascent as pressure is reduced.)
We probably agree that the chance of this sort of minor mis-communication affecting the safety of a diver under a CA's supervision is pretty remote. But given that there COULD be a problem, it would seem to give PADI reasonable grounds to forbid the practice of using CA's from other agencies within their programs.
