This was suggested before on the board. It does indeed stop you being screwed over if you can't finish your contract due to serious illness or being fired or any other valid reason. But most Chinese empolyers wouldn't introduce such a scheme. In fact, many will only REIMBURSE a one-way economy class air ticket between the neareset international airport in China and where that flight will terminate in your homeland. If you don't buy ONE, you'll get zilch. An air fare ALLOWANCE, as such, isn't and actually needn't be offered.
Just to add, beware of contracts that offer air fare allowances as end-of-contract BONUSES. You might end up getting nothing. They should never be classified as bonuses, in any case!