arming A/T before taxi is, 1 to bring it inline with the rest of the boeing family however the most important thing is to reduce the amount of workload on the pilot at the most critical moments eg taxi. to prevent runway incursions.
with the old procedure we had a taxi checklist. which was something like:
select the flaps, check recall, check flight controls, shutdown apu, check trim, check cockpit door is locked. and this was all done while taxiing.
with the new procedures both pilots can direct more of their attention to taxiing.
xetroV: the QRH says that you as captain can perform items by recall that are not recall. however it will be your responsibility if you mess up.
and example of this is a scenario I recently had on the sim.
we were cleared to maintain 4000ft, and maintaining 250kts. the No 2 engine on leveling off wouldn't respond to throttle movements.. ie it was stuck in full T/O thrust. we immediately reduced thrust on No 1 engine but the speed of the aircraft was rapidly rising. we had to according to the checklist take out the QRH find the Engine failure/shutdown NNC and read and do. by that time we would have an overspeed. so instead by recall we just shutdown the engine and then went to the QRH.