The beauty of the MacGuffin in Pulp Fiction is that the whole sould thing and all the clues... were the product of the viewer's imagination. Tarantino has answered questions about the case honestly, he never gave any thought to what was in the briefcase, just that it was something everyone wanted and it could advance the plot.
The way I see it, it loses something if you know what's in the case. The MacGuffin, if named, shouldn't ever being anything truely important to the plot, or anything useful in resolving any problems. That's a widget.
If you've ever seen Psycho, think about the importance of the stolen fourty grand. Most people don't even know there's a stolen fourty grand in the movie, but it's practically the central aspect of the first two acts.
But when it's served it's purpose, in this case leading the film to the Bates Motel, it's discarded from the plot completely. To go to another Tarantino film, and another theory about what's in Jules' briefcase, the stolen diamonds in Resevoir Dogs. Once everyone is at the safe house, the diamonds are irrelevant.
Look at it this way... A MacGuffin is a tool for hunting wildcats in the Scottish Highlands. Since there are no wildcats in the Scottish Highlands, a MacGuffin isn't anything terribly useful, though it may be rare and valueable. Once it's useful to the characters, it becomes a different sort of plot device entirely.