Converting a colour image to grey for professional printing is not quite the same as only printing the black plate.

While this will certainly be useful for printing proof copies, I tend to think it would be much better to use Photoshop, together with a professional plugin such as theimagingfactory's ConvertToBW Pro. This gives you much more control over the delicate balance required to produce a professional greyscale image from a colour one.

I'm not trying to denigrate the author of this software. Kudos for doing the work. I do think, however, that this solution will appeal more to "home" users of InDesign than professional ones.