[Macario]

Try to desaturate a rainbow, then you will see why it doesn´t work. To be able to get difference withing some colors of some densities, you need to work with filter, or after desaturation it will all look just the same gray. Like I wrote before, you cannot put millions of colors into 255 shades of gray just like that, things will even out. So if there is a color that needs to stand out in the conversion, you need to work with filters.

The two examples you showed, are really bad, because in the color version, there is almost no color. They are bothe almost monochrome already

[emalvick]

Stepping back... Desaturating or leaving a luminance channel only can work. That really isn't the problem. The problem is that it won't always work and desaturating only eliminates the potential for color adjustment before the conversion. Afterall, no matter how you do it, you will ultimately desaturate all channels. It is just a matter of what adjustments (if any) you make beforehand.

In my own workflow, I mostly utilize Lightroom and/or Photoshop (via adjustment layers). In either workflow, the initial step is actually a desaturation but the software allows me to "adjust colors" on the desaturated image until I get the black and white image I want. Rarely do I find my initial image to be the perfect black and white rendition I want. They can be close, but I'm almost always adjusting something with the green channel.