The GIL is a problem for exactly two (real) reasons:
1) Some external system mandates the use of threads. This can be the operating system which requires threads for certain types of I/O. Or it can be other applications that are naturally multi-threaded, and want Python to work seamlessly without clear thead-locality of data.
2) The other group is populated by people who need threads because they do not understand state-machines. So threads become the one-stop-shop for all forms of programmatic concurrency, without regard for the overhead they involve.
If I am right in my thinking, this refutes the argument that the GIL is only a concern for those with multiprocessor systems. Of course the MP argument is raised all of the time, but only because it is easy to spout on about requirements instead of features.