>>107
>>97's point is that requiring write(2) to always complete on the first attempt places an undue burden on the OS. If the output device or file cannot accept writes over a certain size, the OS is now obligated to buffer the whole input and fragment it itself, rather than just accepting as much as will fit and relying on the caller handle the rest.
The argument is that this added complexity is pointless as in the general case the reader will end up fragmenting the data anyway because it can't guarantee all of it will fit in the buffer it allocates for read(2).