Why is that ever a good idea? I'm not being aggressive: I'm genuinely curious. It seems to add an unnecessary step to the workflow.
It makes sense for me, but in large part because I use raw plus JPEG mode, and do a lot of basic tasks with the camera's JPEGs, which with my [Olympus] cameras are often completely satisfactory. For example, when only a basic crop of a good camera JPEG is needed, cropping that JPEG in Preview(!) is the quickest way. So I want to separate the files into folders for JPEG and for raw, and import only the latter into Lightroom. For me, shooting raw-only and then having to use Lightroom to convert every image that I want in JPEG format would "add an unnecessary step to the workflow."
I also use tools other than Lightroom, and so want to avoid mixing their output into the folder where Lightroom has "catalogued" raw files --- though I do not know if that would actually be a problem. (Edit: for example, I put albums onto my iPad after arranging them in iPhoto, so want a folder of just JPEGs to drop into iPhoto.)
And the default folder naming and organizing convention used by Lightroom is not to my taste, so I prefer to handle that "filing" step in the Finder before importing to Lightroom.