[RonaldoNZ]

It's to shield emf from that signal wire.

Indeed, couldn't have put it better

Its really quite simple, 0V should always be tied to ground unless you plan on running the circuit isolated for some reason which is just going to create problems with your circuits 0v floating with respect to ground/earth. This is exactly the problem the "serious noise issue" guy had.

So...since the shield and the 0V rail are in fact already (should be) connected the effect of running a separate 0V conductor inside that 0V shield does nothing other than effectively make some strands of that shield be insulated from the rest for the length of the cable run.

Edit to add: Al thanks for that siemens link, its excellent. Damn board ate my two previous replies, perhaps they where conducted to ground.