Including UX designers in user research / testing processes – considerations

I have been asked previously to participate as user testing processes (formal and informal) and in the screening process the fact that I am also a UX designer doesn't seem to have been an exclusion criteria (just thinking about lawyers in jury panels).

I am wondering if others have had experiences when choosing or screening participants for user research or testing activities to deliberately include or exclude UX designers and why? What are the main considerations that you have had to deal with?