most of the schools ive interviewed at could care less. Some schools like ucla i know dont even care and told me so lol. In general i think these letters are one step above useless. Of course you want to go to X school, i mean you applied and interviewed there right? And because you write one sappy letter saying you want to go there for the same generic reasons every other pre med does (research, volunteering, intl opportunities, and community involvement) they are all the sudden going to change their mind?
I only know of a few schools that put a lot of stock on these and thats UCI, vermont, and i think gtown? Im sure there are others, but i would assume for most schools its not going to change much. If i were an adcomm i know i would care less lol. The only time id care is if they not only said they want to go to this school because of X generic pre med reason, but also had a strong reason like they want to be with their wife, kid, fiancee, w/e.