At my department, a thesis defence committee consists of 3-4 professors (including the advisor(s)), one of whom does not belong to the department (so could be pretty much anyone related to the field). It is common for a grad student to know who his thesis defence committee members are going to be quite ahead of the defence date (usually the student will know before copies of his thesis are sent to the members for review).
In rare cases, the student can even assemble his committee together with his advisor -- this makes sense for example in the case where the student wishes to pursue a certain research direction after graduating, and he would like a potential advisor or reference who is an expert in the subfield to take a part in his evaluation.
However, the general policy of the university (and of some faculty members) is that a student is not supposed to have the information on who is included in the committee until the defence.
While I understand some potential reasons for it, such as avoiding bribery in any form, those precautions seem quite far fetched at a small department where everybody would probably know if any foul play was going on. Also, if it really were an issue, there should have been anonymity in other examination-related situation during one's studies.
From a different perspective, I personally would rather not discover that Euler is on my thesis defence committee when I walk into the examination room (and not because of the startling discovery of the existence of zombies).
What reasons, from your experience, lead to such no-transparency policies?
(I am not facing such a situation, the question arises from a case involving a colleague.)