Why Let Tolkien's Letters Have the Last Word?

What people forget when they quote from Tolkien's Letters, in no particular order:

1. Each letter was written to a particular person, for a particular purpose.  So the statements may be true as far as they go, but they may not be the whole truth; they could be selected to create a particular impression.  There may be conscious or unconscious embellishments, shadings, or omissions, both large and small, as suited to the context and purpose of the communication.  (Hat tip: Dr. Verlyn Flieger; perhaps also cf. Emily Dickinson's "Tell all the truth but tell it slant — Success in Circuit lies")   
2. Each letter was written at a particular time. Tolkien's memory of long-ago events (including his own writing process or intentions) is no more reliable than anyone else's.  See, e.g., John Rateliff's History of 'The Hobbit', where he painstakingly investigates the book's actual composition history and reaches a very different conclusion than the timeline Tolkien claimed.
3. An author's intentions and explanations (even if 100% accurately recalled and explained in a later writing, untinged by any agenda) are not necessarily the best guide to interpretation of the text.  Or as Lewis put it : 

"It is the author who intends; the book means. The author's intention is that which, if it is realised, will in his eyes constitute success. [...] Meaning is a much more difficult term. [...] The nearest I have yet got to a definition is something like this: the meaning of a book is the series or system of emotions, reflections, and attitudes produced by reading it. [...] The ideally true or right 'meaning' would be that shared (in some measure) by the largest number of the best readers after repeated and careful readings over several generations, different periods, nationalities, moods, degrees of alertness, private pre-occupations, states of health, spirits and the like cancelling one another out when (this is an important reservation) they cannot be fused so as to enrich one another. [...] As for the many generations, [t]hese serve to enrich the perception of the meaning only so long as the cultural tradition is not lost. [...]  Of a book's meaning, in this sense, its author is not necessarily the best, and is never a perfect, judge."

(On Stories 139-40).

Lewis, C. S. “On Criticism.” On Stories, and Other Essays on Literature, edited by Walter Hooper, 1st ed, Harcourt Brace Jovanovich, 1982, pp. 127-41.