This operation of A/T on App has been discussed at depth before. If memory serves me well (13 years ago) it is this way on B757/767. When I transferred to B733 many asked why we could not fly in 'SPD deselected'. These were old B757/767 pilots. Trouble was the C.P didn't understand the question. However, some had experimented on the line and found it worked just fine. I didn't so have no diect knowledge of it. You can disconnect at flare I supose. Anyway, said C.P asked Boeing. Answer, not recommended. However, from flying these types it seemed Boeing flight dept's don't talk to each other that much. Advancements in later design didn't seem to filter down to earlier types, when suitable. It seems B737 line pilots fall into 2 categories, but Boeing only 1.
Moving on: about manual flight = all on, all off. IMHO that is not always wise. On B733 the TCAS manoeuvre was A/P OFF and follow guidance. Now, the NG is disconnect A/T as well. When a Boeing pilot was asked that surely this introduced a safety (speed control) threat and was a retrograde step, the reply came that Boeing has adopted the philosophy of 'all on or all off all the time' The argument of having seen dangerous developments in the sim with bad speed control, absurd ROC's, forgetting to re-engage A/T after RA, etc. met with little appreciation. Why disconnect A/T if having a CLB RA above 25,000 and then forgetting to add enough thrust; or going full power at low level and hitting 3000fpm in a crowded TMA. DAFT. So, IMHO, Boeing doesn't always have the be all and end all opinion. They changed what was a safe action into a less safe one, so as to conform to a pedantic philosophy. I hear of one airline that teaches manual approaches with A/T engaged until stabalised in final config. I assume they asked Boeing?