From what I've been told,there are oftentimes two distinct reasons why potential Chinese employers ask for photocopies of the identity page of jobseekers' passports. One is to determine whether the jobseeker is black.The other is that the PSB won't authorise the letter of invitation required by the successful candidate to obtain his (or her) work visa without said photocopy.
That said,how much of a threat is it to a passport holder if the personal information displayed on his (or her) passport's identity page should fall into the wrong hands?I'd say not much,unless additional information about things such as personal bank accounts is also available to a would-be fraudster.But I'm no expert re matters of identity theft.So I could be wrong!