SOs are very well placed to see some of the errors that may slip past the guys in the front seats. Taking some criticism in front of the crew is a well established standard. Everyone tries to learn, no matter who made the 'mistake'.
The reason I like having the SOs debrief is that it really forces them to think about what they saw. I'm not interested in being told what went wrong, I can almost certainly see that. I want the 'why'.
Plus of course, if you've invited them to be critical, you'd better be prepared to accept their commentary.....
