So your basically telling me that black and white conversions are only performed for tonality. And coloured pictures aren't?
No, I am telling you (and since you have shot almost a hundred weddings in the past three years you should know) that B&W does not show colors, only varying tones of reflected light. For example in the above shot her forehead is roughly the same tone as the trim on the house, not really appealing. This is why images that are supposed to be B&W in the finished print should be shot looking for reflectance values and not colors.
A red apple may stand out beautifully against the green leaves of a tree in color, but may completely blend in when shot in B&W.
To answer your other questions...
What if the client requested for black and white?
Then before you trip the shutter you need to look at the scene as if it was black and white and adjust accordingly.
what if the artist is just a black and white photographer?
Once again, you need to visualize the scene in B&W and make adjustments accordingly. I have had to do that for over twenty years, the rules of B&W do not change just because people use digital now instead of film. (although I use both)
So turning a really bad colored picture can not be converted to Black and White?
Sure it can! But it will still be a really bad picture. A bad shot is a bad shot, color or B&W. Conversion may add "class" in some people's eyes but it will not improve the shot.
Frankly speaking I wouldn't set rules in the art if I were you...no such thing in my book. Not stepping on your toes Allan but I've done almost a hundred weddings in the last 3 years and surely enough Black and White conversions apply across the board.
I am not setting any rules. Black and white pictures capture the reflected luminance of a scene, not the colors, that is a fact. Different colors can exhibit the same reflected luminance, that is a fact. Therefor what may look good in color, may look like garbage in B&W. The reverse is also true. So to achieve a good shot the photographer must visualize, compose, and shoot for the final output.
Allan
PS. That last sentence is really not mine, you will find something like it in any photography book.