IMHO, no, it doesn't do any harm.
While the transmission may be in neutral,
the tranny computer is still on full alert,
monitoring speed etc
Engaging drive again after coasting a while,
should be uneventful.
You'll probaly be using your brakes more though,
personally I like to let the engine do as much of the braking as possible.
It also does a diesel much good to drive in sport once in a while.
Clears out the soot built up
while pussy footing around in traffic.

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