Benefit of using MacPorts coreutils instead of stock OS X?
The obvious benefit is to recompile the tools with optional features that Apple chose to not enable when they compiled the tools. Also, you can get newer options / patches / bug-fixes if you don't mind the duplication and the fact that sometimes you also get new bugs with the newer code as well.

The downside is that tools like tar and zip know about Mac OS X metadata that the MacPorts versions probably do not.