Aero99,
Your friend in the Caravan is, while technically not an idiot, unwise for providing any encouragement to anyone to fly over gross. Yes, many airplanes will do it, but the proper answer to the question of flying over gross is, "No, I would never intentionally fly over gross."
The weights specified for any aircraft design are not put together haphazardly. These numbers represent a series of factors, and are related to aircraft performance requirements, loads on landing gear and aircraft structure, wing bending moments, potential air loads, etc.
It's not uncommon to hear people state that it's okay to push a little farther than the design limitation for an airplane because it already has "150%" strength built into it. Or that it's got plenty of performance. The party line when I flew 207's was that they would haul whatever went into them (more true of the 206 than the 207, but beside the point). While a pilot in the bush who knows his business can certainly work those airplanes, I don't find it productive or proper to advise others on flying outside published limitations.
Most airplanes have landing weights which are less than takeoff weights, and this is based on the ability to burn off the difference in fuel, or dump it, prior to landing. Most airplanes have ramp weights greater than takeoff weights, because the difference will be burned off in fuel during the taxi. This is acceptable, and within the limtitations of the airplane. However, if one is flying over gross in the hopes of burning off enough fuel to be at gross upon landing, one is engaging in unwise actions.
Published stall speeds and performance information are based on a known quantity, including weight and balance. Exceeding those limits places the prospective performance outside a known envelope. This includes glide performance in the event of power loss. It also places the pilot in jeopardy for his or her certificate if caught; a high liklihood in the event of a mishap. The loss of a pop star in the Carribean recently was publicised as having been caused by an aircraft loaded over gross. This was true, but the weight had little or nothing to do with the accident. However, that's the one thing the media grabbed and ran with. Hardly surprising.
Many aircraft have required inspections for any landings over gross, or hard landings (which can certainly occur with normal proceedures when flying over gross). Manufacturers take it seriously. I observed the failure of a landing gear trunion assembly in a spar box during an inspection once, caused by hard landings, most likely performed over gross (based on other information). Considering what I saw, I was very surprised that the landing gear hadn't failed prior to the inspection. The aircraft was immediately grounded.
Heavier than gross weights impose additional bending loads on wings that are outside the operational design limitations for the airplane. That is in normal flight. Any air loads or gust loads may increase this value beyond the design limitations, even to the point of failure. Depending on the aircraft, other factors may come into play, such as controllability, aerodynamic balance (buzz, flutter), etc. The loading of the aircraft will have a big play in this.
Many aircraft have the capability of being flown over gross, but that's beside the point. Some things are legal but not safe, and others are safe but not legal. As professionals, we strive to ensure that we are both safe, and legal, at all times.
The pilot of the airplane carrying the lions, in the picture shown you by your friend, later said he wouldn't do that again.
Toward your origional question, the weight values are engineering values specific to the design of the aircraft. Unless one is an authorized engineer and test pilot, with the approval of the FAA and the manufacturer, and with the intent and program to develop an STC or expand the origional type certification data, one should respect the published limitations for the airplane. I doubt your friend is paid enough to serve as a test pilot, and based on his comments, he hasn't the experience.