The poem Mythopoeia, from which Walter has already quoted, regarding the splintered light, is opened by the following line:
"To one [C.S. Lewis] who said that myths were lies and therefore worthless, even though 'breathed through silver'."
which could be a refference to a discussion between Lewis and
Tolkien, recounted in "Tolkien: Man and Myth", by Daniel Kennelly:
The above also shows, in my opinion, that if all else fails, we could only discard allegory in LotR only if this work is not a myth.
Moreover, as Tolkien notes in Letter#181 To Michael Straight (drafts):
So, at least two more proofs about the presence of allegory.