Droopystop,
I think the main reason the US military provide both is that it is the best combination to afford the best protection. The helmet provides a good protection from vibration and pressure waves heading toward and through your skull, and extra earplugs just make sense to give maximum possible protection in certain frequencies, especially the high pitch. This effectively covers the whole spectrum of vibration and noise. A very good idea, and I believe if I recall correctly the Australian military do the same, for the same reason.
As far as evidence, here is a link. A study conducted by the Norwegian Aviation Medicine Institute, showing the high levels of vibration and noise in the S92 over other types. Sorry if you can't read Norwegian, but google translate does a fairly good job. It would appear hat the Norwegian authorities are the only ones taking this seriously.
http://www.flymed.no/files/Helkroppsvibrasjoner%20og%20støy.pdf
Also see this thread, with more anecdotal evidence.
Hearing problems and flying the S92