On the topic of procrastination (which was
way up there

) -
Perhaps Tolkien was wrote it this way (consciously or subconsciously) as a reflection of himself. Tolkien was a self-admitted procrastinator, and since he considered himself to be hobbitish, this may have just been the logical way for Frodo to act, combined with the ideas that he preferred to walk in autumn and all that good jazz.
Besides, it makes for a much more interesting story, because if he had not waited then he would not have encountered the Nazgūl, and if he had Gandalf's letter from Bree would not have been there. So maybe the delay is as much for literary purposes as anything else - and then he used it to show us some things about Frodo.