Intermediaries in user research

I’m looking for thoughts, suggestions, references regarding the topic of using “intermediaries” in user research. Whilst purists would argue you should not talk to one person about what another person would do/say/think, in practical terms sometimes that’s all you have to work with.

Personally, I have found that there is often a key person in an organisation that is able to speak with great insight into a particular audience group. For instance, a call centre representative who deals with customers day in and day out, can tell you quite a lot about customers.

I wouldn’t rely solely on this source, but included it as part of my triangulated approach to investigating the target audience.

But what are the caveats? Where is the line after which it’s bad practice? What are your experiences (good and bad)?

Update: I don’t think many people share my terminology wrt “intermediary”. Perhaps “second hand information” is more appropriate. What else do people call this?