I agree with you with some reservation. If someone states that they remember reading a quote and is challenged to source it then by all means the originator would need to research and provide that information or come back and indicate that they couldn't find it. If the originator asks for someone's help to source it and receives that help then fine.
Sometimes providing a source is a waste of time. An example yesterday would be the confusion over what type of seat covers were in the back seat of the Jeep. It never was actually cleared up one way or the other but in my mind it really didn't matter a whole lot. This is because the removal of those covers is circumstantial at best. The actual type of cover is not important because Ricci could have removed them for any number of reasons.
If I am the offender then let me know what specifically you are referring to. I do tend to use my memory of what's been written to make my points.