Section 8.2.1 Header Accept states (new text in 4.01, not present in 4.0):

If the Accept header is specified on an individual request within a batch, then it specifies the acceptable formats for that individual request. Requests within a batch that don’t include the Accept header inherit the acceptable formats of the overall batch request.

This would cause problems for batch requests specifying Accept:application/json with the intention of a "default" for the individual responses once we add a JSON Batch format. Services would not be able to respond with the JSON Batch format if they formerly treated this as a default for the individual requests