having official credentials can sometimes be useful to give us a rough indication of standard/validity. however, it can be equally as misleading at times. there are people with high ranks/quals with crappy abilities, and vice-versa.
i know this isn't exactly where you are coming from with your question, but i guess this is just another POV to consider aside from an instructor's qualifications is their ability as an aikidoka(ist) as well as a teacher.
A piece of paper is no guarantee of quality, that's correct. However, someone who lies about having a piece of paper should tell you all you need to know.