True, Rsync has more features, but Robocopy is still a very useful tool. And it has been included in Windows since Vista.
But with all the problems of rsync and cygwin on Windows, Robocopy is the more logical choice for such tasks. I'm not going to load up explorer.exe in WINE on Linux so I can move some files around with the mouse.
Rsync in cygwin also (severely) messes up your files' and folders' NTFS-permissions when synching against a linux-host, which can lead to things like the meltdown of a complete IIS website/webserver setup since the IIS worker process can no longer access the files it needs.
I've learnt the hard way that you dont use rsync in cygwin for anything remotely crucial ;)