Dear Stephan and Edric,
Thanks for the very informative feedback. To explain the problem, I do automated acceptance testing on code that relies on reproducibility of results. The numerics are expected to be the same no matter which OS, version of Matlab or number of processes. This is the only practical way to pick up what might be subtle errors in the code.
If the default parallel RNG is changed, then test results change in the current simulation code, depending on the Matlab version. This could cause problems with users testing distributed code to be certain it is running correctly and is not corrupted. Results obtained on earlier versions of Matlab won't be the same on later versions.
It seems like there could be a way to solve this through specifying the RNG, but this will require testing with different versions, to make sure changes don't break code run on old Matlab releases. Incidentally, my code is often run by other people, who may not have access to all Matlab versions - and in some cases they may only have Octave.
This is an issue with third world researchers. I have a university license, but not everyone has this. Some researchers prefer to tolerate longer run-times, if they can use older Matlab or open source.
The help is much appreciated -