Really wish it had a 3.5 mm jack for headphones as well. Use the big one for normal work. If the jack breaks inside the socket you switch to the 3.5 mm instead and save the day.
I've gotten rental gear (664) with one of two headphone outputs broken because of this.
Two sockets. One professional and one consumer. The latter will save you when you have to buy an emergency headset in a rural town somewhere in Africa, and the only thing they got is a Casio or some shitty noname brand.
(Yes, I usually bring backup headphones with screw on connectors. Prepared for anything)