some correspondence i received from ucsd today (i'm in the acceptable pool)
""""
Concerning the issue of "letters of interest" or "letters of intent" I
tend to share your feeling*. Admissions committees are very busy, get a
lot of information and assume that students that applied to their
institution are interested in attending there. Letters bombarding them with
statements of how great they match with the school and how much they
want to go there get read, but don't hold much weight. Weekly letters to
that effect are inappropriate and a burden and likely will do more harm
then good. The times it is appropriate to write to the admissions
office is to update them on significant changes to their file and one, such
as yours, at a late stage in the process, reaffirming interest in the
school is not looked at askance.
""""
*'my feeling' being that applicants probably shouldn't send so many letters... mine was mainly of the update sort/interest reaffirmation