Should all recommendation letters be research- or teaching-related? I am applying for junior faculty positions at research universities.  Most job postings request three references (either just names and contact details, or actual letters of recommendation), and I've already secured these from the three full professors who can best attest, in different aspects, to my research and teaching skills.  One of these professors was my thesis advisor, and the other two are faculty at other universities with whom I have worked closely.

A few job postings ask for five references.  Now, I could approach two further senior academics who could also vouch for my research and teaching abilities, though there's no one I have worked with nearly as closely or as recently as my three top references.  I have no doubt that they could supply positive references, and while their names alone might carry some weight, they wouldn't be able to write about me in as much detail.

On the other hand, I do have some notable collaborations with professors on work other than research and teaching—namely, on outreach and knowledge transfer activities.  That is, we have cooperated on projects aimed at disseminating scientific knowledge and research results (not necessarily our own) to the general public and/or industry.  Would getting a reference from one of these professors be of benefit for a faculty application?  Or would most selection committees be looking exclusively for research and teaching references?
I see relatively little value in having additional letters that all speak to the same topics. If you think that the existing letter writers already cover your research in positive enough light that your research credentials are unquestioned, then having additional letters cover other areas is certainly a plus as it shows you breadth as a professional.

It would probably help if the other letters could at least very briefly mention the topic of research so as to not look like you're scraping the bottom of the barrel in coming up with more letters. This could be done by these letters containing statements of a form like this: 


  I have known Dr. Psychonaut for several years and have in particular observed her greatly grow in her research since I first met her. I have also seen her give a number of marvelously executed talks on her research. That said, I know that several of my colleagues are already writing letters that address her research credentials in great detail; I will therefore rather focus on a separate area in which I have collaborated with Dr. Psychonaut -- namely, knowledge transfer -- and for which I know that my opinion is unique.


What I'm trying to say is that the letter should be written in a way to make clear that the letter writer could be talking about your research, but consciously chooses not to because they have this other thing they know and only they can talk about. This avoids the impression that there are only three people who can comment on your research and teaching, and that you needed to go out of your way to find a fourth and fifth letter writer.