I don't really have an argument with that view, Cheryl. Diffusion at the printing stage adds softening to the whole image (unless obviously you use a diffuser with a hole in it), and has a bit of a muting effect. This is just the unavoidable consequence of diffusing light from a negative where the shadows are going to spill over a bit into the highlights. This can look good and serve some kinds of purposes and has one kind of flexibility in that you can obviously choose whether to print sharp or soft.
In the Mapplethorpe prints that have diffusion at the print stage, it looks like he (or his printer) used it with more elderly subjects to soften some wrinkles, but then let it go just across the line, so they would look a bit sinister. I think it's very effective.
A soft focus lens has a different kind of flexibility, because you can vary the effect by changing the aperture or changing the focal point. Some SF lenses (Veritar, Imagon) create a kind of overall diffusion, but the ones I like are both sharp and soft at the same time, and have something of a glow. You can't really get the Verito-glow by diffusion at the printing stage, and no kind of diffusion screen either on the taking lens or the enlarging lens has the effect of both sharp and soft like a Heliar wide open or a Verito around f:5.6 or f:8.
My point is just that each of these approaches is a specific effect, and one is not really a substitute for another, and which one you choose is going to have an overall impact on your style, just like having a preference for, say, softboxes as opposed to umbrellas, or Tri-X as opposed to HP-5+, or Forte Polywarmtone as opposed to Gallerie. They're all interesting, just not all the same.