sorry to continue the drift but, I am still looking for a rationale answer to the question: "what added safety provides to initiate the after start checklist only after the clear signal from the ground crew instead of doing the after start checklist, wait for the clear signal and then ask for taxi clearance?
One reason would be that with a well-designed procedure/checklist you have two chances at getting it right: the procedure (check crew clear) and the checklist (are the crew clear?).
You have to omit both of these to be caught out, as opposed to just a single error in skipping straight to taxi clearance having completed a checklist that didn’t include “crew clear” as an item...