The hidden costs of GPL software?

      If the GPL were not so tight on this point, someone could
commercialize a GUI for R without having to offer their source code
under the GPL.

There is nothing in GPL to stop a commercial GUI for R.  Have a look at 
what Apple do.  They have a complete commercial GUI and numerous 
applications built on a an almost completely GPL'ed operating system.  
There are loads of shareware GUIs which drive GPL utilities.  Most 
obviously there are plenty of commercial apps which run on GNU Linux.

Bill Northcott