Is there some articulatory reason behind why we choose to preface consonant sounds with the article a and vowel sounds with an? The reasoning I've read in the comments somewhere, I don't remember exactly where, is that we are trying to avoid consecutive vowel sounds, but why exactly are we trying to avoid these consecutive vowels? Are they harder to articulate for some reason? (If so, can that be explained as precisely as possible?) Are there many cases that can pointed to in English where we similarly avoid consecutive vowel sounds? Why not dipthongnize the vowels? Is the whole thing mere idiosyncrasy on English's part?
"Are there many cases that can pointed to in English where we similarly avoid consecutive vowel sounds?" I'm not English, but the a/an questions reminds me of the word 'idea', that it's often pronounced as if followed by a sort of 'r' prefacing vowel sounds (i.e. 'idea of...'): [ai'di:ə]. According to the IPA, 'ə' is used as in British English 'butter'. As you say, it must due to a co-articulatory reason (vowels are open, palatal sounds, and it may be somewhat difficult, if not unpleasant, to pronounce two of them in a sequence and the speaker seems to need an 'infix' to interrupt the continuum). But then why written English follows the same rule? There it's not an articulatory problem... F. A. P.S. I am Italian, but my last name has a foreign orygin and it's often mispronounced. The interesting fact is that the first letter is 'A', followed by a 'd': well, almost every Italian inserts 'n' between the two, apparently for euphonic reasons.
Try saying "a idea" (a pronounced with schwa sound) in a lazy and somewhat sleepy tone of voice. That should explain to any listeners the reason for it. Once the speaker is awake and tries to say it distinctly, he will probably also find out why it is that way. If you are talking fast, it is almost impossible to hear. If you don't get what I mean, it is rather hard to say it and still be distinct.