There must be a drawback to it, but I don't see what? If the opponent tries to break the gap, he runs the risk of getting jabbed or counted with a cross. If on the other the hands are tucked in and around the face, as classicaly taught, it's much easier to for the opponent to charge in either on the body or the face, with good footwork.
Whenever I use conors guard in boxing sparring, the opponent gets confused and often countered more often.
Whenever I use conors guard in boxing sparring, the opponent gets confused and often countered more often.