“No one imagines that a symphony is supposed to improve as it goes along, or that the whole object of playing is to reach the finale. The point of music is discovered in every moment of playing and listening to it. It is the same, I feel, with the greater part of our lives, and if we are unduly absorbed in improving them we may forget altogether to live them.“
I think you've missed the point of the quote. It's not an analogy to prose, per se, but merely stating that you don't need to just "get to the point" and, instead, can enjoy a work throughout. A symphony isn't inherently good, it was just the metaphor of choice Alan Watts chose to convey that we should be able to enjoy living our lives without constantly thinking of constant improvement.
That said, I do also think you can apply it to the enjoyment of prose in that you don't need to read it, tapping your fingers, waiting for some climax and then a minor denouement, expressing frustration if "the point" seems to be taking to long to get to. Certainly there is a lot of bad writing that can be overly verbose/messy/in need of editing/etc. and, depending on the nature of the writing, attempting to write "artful" prose can be a misstep. But, often, I find that you can find great pieces of prose in an essay/article/novel/etc. that are well-composed, sometimes profound, and a general joy to read. Though, judging by many of the comments in this thread, many don't care to read that way.
Alan Watts