OS 'Porting' was old when Adam was a pup.  It's all about altering an existing GUI to be different.  If that 'different' already exists as an alternate operating system then so be it.  BeOS, Linux, Amiga, OS2, Mac, and more obscure GUI-based OS's have all been emulated at one time or another.
What REALLY got strange was when various Alternate shell users [Litestep] actually went out of their way to emulate the shell they were replacing.....an Explorer theme for Litestep.....no THAT was the weird bit...aka why bother?
But, as with all skinning/customizing...it's because you CAN....
The only time it gets silly in a cumulative sense is when every man and his dog does his OWN interpretation of a 'port' and we end up with a squillion versions of 'Longhorn'....