Many- I would venture to say most- of boxing's great defensive fighters fought with a low left hand and in terms of "proper" technical boxing using the shoulder to defend against the right hand is the superior technique as it provides better counter-punchingopportunities and it leaves both hands free to counter. Also there is less damage done tan there is when putting one's left glove against one's head and blocking the right. The idea is to avoid or to deflect as much impact as is possible.
If you watch tpe of Charley Burley, Archie Moore, Walcott, Charles, even James Toney they jab from the hip and it is anything but a light flicking punch. Eddie Futch was discussing the great Holman Williams in an interview and made reference to his rising jab, which hedescribed as a sort of "back handed uppercut." Tape of the fighters that utilized this punch will show that while the left was low they carried their chins well behind the left houlder, fought out of a semi-crouch and had their left hip and shoulder so far forward- towards their opponent- as to be almost sideways.
This makes one very hard to hit with right hands, and makes an opponent walk around and into your right hand if he tries to land his hook. The 'rising jab' comes into play as the opponent jabs; either slip or parry so that his jab goes over your left shoulder, stepping in with the left foot by slightly pushing off the right. The left glove, the left arm straight, comes straight u7p from the hip, timed to meet the foe's cin as he steps in behind the jab you made him miss. This is a very jolting blow, and over time causes considerable damage.
I would also argue that , if done in the manner described above it makes one less vulnerable to right hands, due to the stance and the crouch. In fact it lends itself to drawing and countering the right with a return right to head or body.