Disarming when still moving when the pax are seated means the exits are completely clear and there are no distractions so the doors can be disarmed and (depending on airline) cross checked with the door opposite. Should anything happen between disarming and getting steps to the doors, rearming the doors in the bus takes a second.
A certain A320 operator in Europe did this a few years back and to my knowledge has not had an accidental slide deployment since. I believe it was found that passengers got in the way and also distracted the crew. So they overhauled their disarming procedures, and a checked to confirm that they are disarmed, and once again double checked before opening each door. Each time a door is opened (in normal ops) two crew are required.
Do Qantas group airlines cross check their doors? I heard that they stopped doing that. And do Jetstar use 2L for boarding on the A321?