Oh, that's a provocative heading, isn't it? I could have written something like: "What would Manuel García think about a lip trill?" but that would have been a cop-out. Sometimes you just have to have the courage of your conviction, right? And my conviction is that lip trills are stupid. Why? I can think of several reasons, the most important being that a lip trill doesn't teach the singer anything about vowels. Nor does it teach the singer how to breathe. Let's take the latter point first.
What happens when the singer performs a lip trill? Most often, the rib cage falls rapidly like a wheezy bag. And if it doesn't, that is because it is being held up artificially. Neither is desirable. This leads to my first point.
The Old School was fanatical about 'pure' vowels based on Italian tonal values. When you obtain them, the rib cage is observed to refrain from wheezing or collapsing like a house of cards. Why? The ear leads the body to do everything to keep pure vowels going. 'Support' is a result, not a cause.
There is also the matter of the lip trill occluding the wrong end of the vocal tract. In my last post, Mathilde Marchesi wrote about the student learning to 'close' the vocal folds after a few months of training. I take her to mean that the singer should speak as they sing. Does this mean with affected tone and diction? Nope. She simply means that the vocal folds should adduct (see: I can use scientific terms, my head isn't always in the 19th century). What can enable this? You are close to the mark when every vowel has the vibrancy of /i/ (which brings the folds together), the openness of /a/, and the roundness of /u/. A compound vowel, the result is heard at the level of the eyes, while the throat is heard as being 'open.'
I think lip trills are a waste of time. They postpone the most necessary skill, which is to pay attention to your vowels. No, not your tone. Your vowels. In the end, whether you are amateur or professional, that's what you have to learn to do.