[mwcremer]

I think the thinking here is that the sender may not be able to compute the byte size of the object a priori. Think HTTP chunked encoding.

[ghoul2]

I understand that is a concern in many situations. The problem here though is that you don't get the "streaming" benefits in any case: you still have to include the length-in-number-of-items of the compound type and the lengths of each individual member item in any case.