I doubt large families, or those with new children, strange pets (Like the aforementioned snake), or people with very particular needs are going to be able to clear out in a jiffy. Getting everyone out of my house generally takes longer then that to go to dinner!
However, I, like others, perfer if the owners are not home. Sorta strange to poke around basements trying to find signs of water damage or funky wiring with people hovering over you. However, I think I have only seen one house which was not empty, and I generally see houses during the Open House things realtors do here. Like others, I haven't seen cases where the people who owned the house were present.
As for feedback, not only do people give their agents feedback on what they want, pictures seem to be completely and utterly unhelpful. I've seen brochures for beautiful, antique homes with gorgeous wood paneling. Then you get to the house and its only two rooms. One house managed to hide the fact that the kitchen was spread across three rooms.