There is no practical reason to disconnect the A/T when hand flying
You are asking for trouble if you are flying half manual half automatics in the 737. It is untidy and can cause unwanted pitching movements as the AT tries to maintain the selected speed. If Boeing recommend a procedure - in this case turn off the AT if manually flying except for the climb - then only automation dependant pilots will balk at the recommended procedure. That is usually because they lack the self confidence to hand fly without the crutch of an autothrottle engaged.
