One very good use case for implementing it is this:
Lets say you have a multi-site server with 3 secure sites. You have 3 groups created, one for each of the different sites maintainers. All of the files in all of the sites need to be owned by the apache user so that apache can read and write to them (drupal/wordpress, etc).
If the setuid but worked like the setgid bit for directories permissions would look something like this:
/var/www/sitea - apache:groupa rwS rwS ---
/var/www/siteb - apache:groupb rwS rwS ---
/var/www/sitec - apache:groupc rwS rwS ---
This way each group of maintainers had access to see and touch only their content but the web server user apache could serve all content and there is no need for users to worry about changing the ownership of uploaded files.
Another use case is for Anonymous ftp/http or even sftp/ssh uploads. The group and GID on the upload directory would be root and so would the owner and UID. The other permissions would be
-wx. This would allow everyone WRITE access to the upload directory but they could not read anything once it was uploaded and root would own all files newly created.
So there are two perfectly good and valid use cases for enabling the UID functionality on directories to match the GID bit.