Over at swampratknifeworks.com, there's a SAR guy who swears by the mag block / flint. His main point seems to be that the magnesium block (which is really acting as a tinder, not a spark source) will get a fire started with damp or wet fuel nothing else short of a match or lighter will ignite. If you can manage to get a big enough pile of Mg shavings together, you have a very hot fire for a few seconds.

Seems to me, the Mg block and the Magfire firesteel are complimentary. The Magfire is a better firesteel for those times that a fire steel makes sense. View the Mg block as a special purpose tinder, maybe.