People are always philosophically arguing back and forth on one subject or another about seeing things in terms of "black and white" versus seeing them as a spectrum. The study of singing is no stranger to this. I myself have always been of the opinion that both things are true. If there is a spectrum, at some point on that spectrum you must come to true black and true white.
Regarding the study and act of singing, I believe that there is indeed a spectrum. And yet, to get a little bit "black and white", I admit to not being overly fond of the relativism to which singing, as a physical discipline, can be prone. I tend to believe that there is not one spectrum, but two (this is oversimplified, but will serve my present purpose):
Black & White #1 - I believe that there are singers who have a good, healthy idea in their mind about singing
Spectrum #1 - I believe these people fall somewhere on a spectrum of discipline and developed skill. Some are more developed and some are less, but they are aiming in a healthy direction.
Black & White #2 - I also believe there are people who have a wrong idea about singing
Spectrum #2 - I believe that among these people are those who fall somewhere on a spectrum of functionality. Some are barely functional at all, but there are those who in spite of a wrong idea may nevertheless stumble onto a reasonably correct physical action, or some proportion of physical action that minimally impedes their objective.
-- Reminder: As I said, this is an oversimplification. One could just as easily ask if there is a third, overarching spectrum of correct/incorrect ideas about singing. But let us, for the following point, stick with only the two--
As a result of this, I believe there are singers who can resemble each other greatly in finished product, and yet whose "technical" goals are so different that one can still consider one to be a "better" singer than the other. Time will gradually separate them according to long-term effectiveness and health. This is ultimately what makes the difference between a good singer having a bad day, and bad singer having a good day. They might sound, to all intents and purposes, very much the same. There is no pretending that we all haven't observed both phenomena. It is no different with the aspiring singer who has few native bad habits, but poor instruction meeting in the middle with the person who has more issues to overcome but is receiving good instruction. In spite of the similarity of appearance, it does not make them the same.
I fear the climate of relativism that says that it not only is the same or just as valid, but encourages shortcuts to it's achievement. I think it undermines both the art of singing as well as the vocal health of the singer, whether one is an opera singer, a crooner, or a yodeler. What's in the mind is of great importance, and not just to the endurance and effectiveness of the individual singer, but also to the educational legacy. The exterior may appear shiny for a time, but if the inside doesn't match…. sooner or later, it will begin to show.