Wing Chun uses an upright posture because, as I said in the other thread, it is about keeping your weight behind the punch. In order to do a WC straight punch, with minimal body English, "from the heart" with power starting from the ground, you need a "straight" structure. This also applies to defense.
Picture the hips as a "choke point". If you are bent at the waist and turning the hips to execute a punch, then the punch really starts at the waist/hips. In terms of defense, if an incoming attack is blocked and/or deflected and you are leaning forward from the waist/hips, the energy gets tied up at that same choke point and so throws you off balance and you lose your center. However, if you have what amounts to a straight line to the ground, the entire body strikes with a WC "straight punch", like the video I posted in the other thread. In defense the incoming energy goes to the ground so you have a better chance of maintaining balance.
Also, if you know what you are doing, you do NOT stay in the pocket. In my school we have 3 main axioms for WC.
1. Simultaneous attack and defense.
2. Never meet force with force (there are no blocks, they are deflections accompanied by relief and/or t-steps to further dissipate energy)
3. Attack from the BLIND side.
3 In terms of # 3, If you see two people who actually know WC it is dizzying because each is constantly trying to attack from a flank and so they will both essentially be walking in circles as they fight (though if you actually watch the foot work, its all angles.)