I've used a DLSR to check exposure and lighting on Medium format shoots,
One of the problems with using a DSLR as a light meter is the difference in sensitivity of the highlights. Film can resolve much more in the hightlights than a digital camera can, and can have a much more limited range as well (for B&W). This could lead one to misjudge the exposure; the digital might show blown highlights, where film could resolve something in it. Additionally, some digital cameras - especially Nikon's - lean towards underexposure. Not a real problem with B&W but a potential big problem with color transparencies.