Wagtail
Horse servant
I have had some (not a lot) of experience with pinch boots. I had on heart do not believe these are abusive to the horse if used in moderation and for the intended purpose. I do not believe they hurt horses although there is no strong evidence to either support or defy that statement. I do believe though that it can be as detrimental to some horses jump as it is positive to others. The FEI are in a very strong phase of cutting out dodgy practise and these boots have been on their radar for a while, if they were abusive they would be banned. I do perhaps think there is a case for banning them on a performance enhancing type argument.
If they do not hurt the horse (and causing discomfort is hurting in my book), then why would they make them snap up their legs more or even as you say, have a detrimental effect on some horses?