People and businesses buy negative interest bonds because there is no better alternative use of their cash horde. Storing bills is cumbersome, and for a Chinese investor, nothing may be safer or cheaper than a bond issued by the Chinese government, even if denominated in a foreign currency.'

Yes, a currency play may make the interest rate unimportant, but even in one's home country, the cost of owning cash may exceed what one would have to pay the government to borrow it.