Two sets of thoughts here. I don't know how Tolkien felt about dipthongs,
or about their prevalence in Finnish or Welsh, but my personal opinion,
influenced by choral singing for a number of years, is that dipthongs are less
attractive than pure vowels. In formal singing in English, we have to go
to great lengths to get around dipthong sounds because they don't sound very
good when sung. My choir director is constantly reminding us to hold out
the first, "pure" sound of the dipthong and barely bring in the second sound at
the last possible instant. And sometimes, we completely change the
pronunciation of the vowel. Just last night, we were working on a song
containing the word "drown", which we were told to pronounce more like
"drah-oon", with the "oon" coming again at the last possible instant. So
that would seem to be an indicator, at least among Western musicians!
As for Elvish, I had to re-read what you said about the "number" of
dipthongs. Are you referring to the number of ways of writing the
dipthongs, as opposed to the number of symbols for the pure vowels? I
don't have the info in front of me, so I can't check on that (and my Tengwar
aren't outstanding), so that may indeed be the case. However, in Quenya
at least, it doesn't seem that dipthongs are used very often as compared
to pure vowels. I'm not really an expert in Quenya (and not at all in
Sindarin), but I did a little experiment, and tried to count the number of
dipthongs used in the "Ai laurie lantar lassi..." poem. Going from memory
on the poem, I came up with only eight words with dipthongs(ai, laurie, luini,
airetari, oiolosseo, caita, oiale, nai). I suspect in an English poem of
comparable length there would be many more dipthongs. A similar
experiment for Sindarin using "A Elbereth Gilthoniel" gave four dipthongs in
this admittedly short poem: chaered, Fanuilos, aear, aeron.
My conclusion on Tolkien's usage of dipthongs in Elvish is that he liked some
sounds enough to use them, but he didn't want them to be anywhere near as
common as they are in
English.
