I'd say the big boys do, Y, MSN, G... in checking my logs I have some folders blocked, and they haven't crawled those pages... now it could be they haven't found it though unlikely that they have found other pages put up way afterwards... so I'd say they do. It is an easy file to put up and while the nasty one's don't follow it, still, imho, a valid file to have.

just my opinion
Ian