I haven't been able to find a clear explantion of why non-deterministic functions were considered a problem, just a few hints that it had something to do with optimization or execution. My guess is that additional infrastructure was needed to ensure that non-deterministic functions are always recognized as such so that the execution engine never evaluates the function more than once per row. It appears (though I can't guarantee it) that non-deterministic user defined functions will be supported in the next major release, but must be explicitly declared as non-deterministic.
-bret