Is it common to seek outside letters for the third-year renewal of an assistant professor? I often write promotion letters for faculty going up for tenure or promotion to full professor. For those who are unaware, this involves reviewing a candidate's entire portfolio including research statement, publications, teaching, funding, etc., and cannot really be done properly without reading several of the candidate's papers. 

Recently I received a request of a new sort: I was asked to go through the entire process associated with a tenure letter, simply for a third-year review of a tenure track assistant professor at another institution. (If it matters, the request is from a good R1 school, but not a super-elite ivy or equivalent.)

To me this seems a terrible practice. It is already a ridiculous waste of time that some schools ask for 15 or more promotion letters. Figure that letter each takes a minimum of one day for a well-established senior professor to write.  Could the marginal information provided by the 15th or even the 10th letter possibly be worth that much of the community's time? We are already suffocating under our peer review obligations; adding the huge additional burden of writing promotion letters for routine reappointments strikes me as ridiculous. Yet I'm loathe to refuse, lest I hurt the candidate. 

Is this a practice that others are seeing in their fields, or is this some dean's stupid idea that is being forced upon a single unfortunate college?

Edit: If this is uncommon, we should nip in the bud. Bureaucracy, like entropy, is monotone increasing in this particular universe. Thoughts on what I should do would also be appreciated, though I suppose that's technically a separate question.
I don't know whether it is common, but a version of it, a 4th-year review, exists (determined by department pattern of administration) at Ohio State, and it's actually useful. It gives the candidate a good feel for what the actual tenure review will be like the next year, and alerts them to problems that can be remedied. Letter writers are usually asked to update their letter for the real review (though not if they wrote a crappy letter for the 4th year review). Similar letter-inflation exists in Alaska, Vanderbilt.

I take the point that this is a form of bureaucracy- and service-inflation, but it actually gives reviewers a longer time to read the papers, in case they are not familiar with the candidate's work. The problem-detecting function is quite valuable, I would say.