Standing is preferable because it gets your body out of the white water. You take less of a beating and you don't get knocked off so easily.
Sometimes it takes too long to get to your feet and get paddling so lying down or kneeling is necessary just to get going and get out of the impact zone. Same if you're too tired to get to your feet.
It's often easier to belly catch a broken wave in, get to your feet and ride out of the impact zone or wait inside for a lull to paddle out in.
To all the crew who hang sh1t on people who don't stand ... add a smiley to it or f@ck off to a forum with the mindless shortboarders and grumpy old mal riders. I'll surf how I wanna surf and you can all blow it out your collective arses.