It will do no harm. In fact, it was only at the 11th hour of the most recent revision of the Wiring Regulations that a requirement for all installations (including PMEW/TN-C-S ones) to have an earth rod was dropped - so I strongly suspect that it will re-appear before too long!It's just as a belt and braces approach. What do you think?
It's no different (or 'worse') than having an incoming metal (e.g. water or gas) supply pipe connected ('bonded') to the installations 'earth' - and that is definitely already very much 'required' by regs. Such incoming 'earthed' pipes are known as 'extraneous conductive parts', and if there is some other earthing system, an earth rod is simply another extraneous-c-p.
Having said that, the rod is unlikley to achieve much. The only real 'risk' with a PME/TN-C-S system is that there will be a 'lost neutral' in the supply network, leading to the potential of the neutral and 'earth' within the installation to rise to considerably above true earth potential, However,, as eric has pointed out, the impedance of any domestic earth rod is nearly always going to be far too high to make much difference in that situation.
Kind Regards, John