While I've never tested it myself, I have read in a couple of different books about accurizing the AR15 & read on forums with people who have a lot more experience than myself that the clamp on style of gas block is preferred if you are trying to get every bit of performance out of your barrel. Also gunsmiths who make a living in the US by selling super accurate AR15's all seem to use the clamp on style gas blocks.
Being a machinist apprentice and having spent more time in a Dimension Metrology lab than I'd like, I can see where the claims come from. The set screw would imply a slight bending force in the barrel, causing perhaps a POI shift. I bet for 99.5% of AR15 users this would not be an issue at all, but if you are trying to make the smallest groups possible it is something to consider.