the armrests need to be down to assist passengers in leaving their seats quickly - almost, in effect, a handhold.
Now I understand! I always find it easier to get out of my seat by pulling the arm rest up so it's out of the way. Of course that is normal gate conditions so I just scoot over without bumping me noggin on the luggage compartment. In an emergency however I can see how the down arm rest could act as an extra lever to assist me springing out of my seat, scrambling over the poor sod to my left or right (as I'm in the window seat), and making sure I trample over them on the way to the emergency exit!
Ozzy