In today's NY Times opinion piece, Early Admissions, Karin Cook spoofs the preschool first choice letter. Focusing a microscope on the first choice letter as a symbol for the whole preschool application process, it illustrates how basically normal people seem to turn into total wack-jobs when faced with applying to NYC preschools in for their kids.
It's not that the parents who write first choice letters are nut cases, it's that the pressure of the process seems to turn any normal New Yorker into a nut - searching for every angle they can to make a connection with the school.
So is a first choice letter even useful? If it is a simple reassurance that if you are accepted, your child will attend, then it may have some small value - allowing the school to get a better count of actual spots, but even this is questionable.
Certainly, first choice letters do not have a fraction of the mythical power that people seem to think they do. If a connection is going to be made it is much more likely to be made during the interview where there could be some chemistry between you or your child and the representative from the school. In most cases, being normal and charming will give your child a better chance than anything extraordinary that people hypothesize schools are looking for in applicants (wealth, power, working or non-working, etc).
So, save the showing off for anonymous message boards.