An even, and regular vibrato in a singer’s voice is indeed evidence of a well trained voice, but not because it is a highly technical and developed SKILL. Vibrato is a natural occurance in the voice, something that will manifest itself when the muscles that are used in singing are in balance and relaxed–that is, when the singer’s sound production is healthy, with plenty of air supply. An opera singer for example wants to have a regular and constant (except for effect) vibrato throughout his or her range, because it demonstrates mastery of their classical technique. It is possible to sing without vibrato (obviously), but “straight tone” singing takes more effort and can strain the voice if not managed correctly. In fact, many pop singers are in serious trouble when it comes to the health of their vocal cords (ie: Britney Spears being out for a couple weeks with a vocal node…Celine Dion having to speak in sign language after a concert.) So when you hear Whitney Houston hold the straight note, then begin vibrato afterward…her voice has not reached balance until the point that her vibrato began. There are at least three theories on why vibrato occurs in the human voice, but I won’t be annoying and try to explain. (Unless someone wants me to.) 
--Leejae
The staff report to which the OP refers, may be found here, Why do singers use vibrato? Dated: 03-Dec-1999
Welcome to the SDMB, LEEJAE. A link to the Staff Report is appreciated. It is Why do singers use vibrato?
Missed it by that much.
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