What advantages does an obtrusive, baulky professional digital camera have over my pocket-sized Sony Cyber-shot DSC HX5V digital camera? There must be some reason apart from prestige for professional...
Interchangeable lenses. A wide variety of lenses are available for the pro bodies, ranging from ultra-wide fisheyes to super-long telephotos. Also macro lenses, defocus lenses, tilt-and-shift lenses, and others providing capabilities you just don't get on a compact camera.
Larger sensor. Smaller cameras by necessity have smaller image sensors. Packing the same number of megapixels into a smaller sensor means that each pixel is smaller. This cuts down on its light-gathering ability. Larger sensors are more sensitive to light. Trying to match the higher sensitivity on a small sensor leads to much more digital noise. In general, if you take the exact same picture with a pro camera and a compact camera, the picture from the pro camera will be of higher quality. Of course, if you never print your pictures larger than 4x6 or 5x7, you may not see the difference. But if you make a three-foot poster, you'll see the difference.
Larger sensors also allow the photographer to reduce the depth of field. One of the factors controlling depth of field is the lens focal length. Small sensors need shorter lenses, and short lenses have wide depth of field.
Speed of use. Compact cameras tend to be designed for the non-professional. They tend to be easy to use, automating many features and providing menus to hold the user's hand while guiding him/her through the non-automatic stuff. Pros don't need the hand-holding. They want to be able to control everything, or at least be able to if they need it. Rather than making choices in a menu, it's faster to just press a button and turn a dial. Of course, with all the settings possible, you need more buttons and controls, and you need a bigger camera to hold them all.
Unless I've left out something, I think those are the main reasons pros put up with the expense, bulk, and weight of their cameras and equipment. If anything I've said is unclear, please feel to free to respond with any additional questions.