inselpeter contends: On the matter of the three additional vowels, in fact they are, of course, diphthongs
This is not quite true as *I understand it. They are vowels unto themselves. The fact that descriptions to English speakers of the production of them involves reference to two different vowels is irrelevant. The ä is, in normal speech, indistinguishable from the e. The ö is pronounced with the lips formed as to pronounce an o but with the tongue in more of an e position. The ü is pronounced with the lips forming a u but the tongue trying to say i.
As for pure vowels vs. diphthongs, this is a matter that varies from language to language. Speaking also as a singer, I'll say that one of the more difficult tasks of an amateur choral director is that of getting singers to sing pure vowels when singing in a language such as Church Latin that has the pure vowels. Another difficult task is to get the singers to change the vowel sound in diphthongs at the right place.
