Some colleagues and I recently discussed whether you should agree to write a tenure/promotion letter for someone whose work you don't know particularly well, if at all.
One colleague argued that if you haven't heard of someone and you have never read a paper by them before, you shouldn't write a letter because you will have to say that you don't know the candidate and this will likely be interpreted to mean that they are not visible in their professional community. Also, if you haven't heard of someone in a fairly specialized field, can their research really be any good? And then there's all the work of reading the papers you haven't read before. This colleague refuses to write letters for people whose work is completely unknown to him.
Another argued that if you don't know someone, then you can be objective and your letter might have more weight than that written by the candidate's best friends and collaborators who can write detailed letters from the point of view of people closely involved with the research.
Another agreed, saying that he had recently written a letter for someone whose work he didn't know, and in the process of reading papers by the candidate, realized he should know about this person, whose research he found to be interesting and excellent. Being unaware of someone's work can also be a reflection of the letter writer's level of awareness of the field.
It is important to note that this discussion took place in the context of professors at a research university that values high visibility, international reputation, publication in high impact journals etc. etc. We are expected to be known in our field; hence, our discussion, which may not be relevant to other types of institutions.
The discussion took place among colleagues from a wide variety of STEM fields. I suspect that some of my colleagues' opinions relate in part to how likely it is that you have encountered most others who are working in your specific field of interest, at the very least reading a paper or seeing a conference presentation by the people active in your field. In some fields this is expected, in others perhaps it is less common.
I think it is also important to note that I am not talking about whether a letter writer knows someone personally or not. I am talking about whether you know of someone's work, e.g. through publications, proposal reviews, or conference presentation.
I don't have a strong opinion about the issue based on personal experience because so far I haven't been asked to write a letter for anyone whose work I didn't know at all, though in a few cases I had to delve into the literature quite a bit so that I could write a substantive letter.
There was one case in which I was asked to write a letter for a promotion to professor, and I said no, I didn't have time. In fact, I wasn't familiar with anything this person had written in the past 6+ years. I assumed the candidate had been publishing in another field and I did not have time at that point to do a good job reading an unfamiliar body of literature. In that case, I was only given a few weeks to write the letter, which is not enough time in general and was impossible in that case owing to the coincidence in time with a grant proposal deadline and some travel, so I did not feel too bad about saying no. It turns out that this person hadn't been publishing much, so I am very glad that I said no, especially if I had to answer that pesky question of "Would the candidate be promoted at your institution?" (which is only a fair question if the institutions are truly comparable in academic environment , resources, and expectations).
In general, I think that if I were asked to write a letter for someone whose work I didn't know, I would spend a bit of time poking around to see if my lack of awareness was my own fault or reflected a true lack of visibility by the candidate. In the latter case, I would then have to see if I had time to do justice to a letter by delving into publications etc., and then I would decide whether to accept or decline. I guess you can say that, unlike some of my colleagues, I don't have a philosophy of definitely agreeing or declining based only on the fact of not being aware of the candidate's work.
So, if you are at a major research university and are asked to write a tenure/promotion letter for someone whose work you don't know -- perhaps even someone you have never heard of -- would you write the letter or not?