To address the original question, there is no requirement in the GUE system that a diver carry ditchable weight. Most of us diving cold water end up with some, as Richard has observed, simply because it's too hard to figure out where to put enough weight for diving in thick undergarments without using a belt or harness. But in warmer water, if the rig is balanced, it is just fine to have none.
If you think about it, when do you need ditchable weight? It is very dangerous to jettison significant weight at depth, because it becomes difficult to impossible to control the subsequent ascent. Most problems you can posit (entanglement, for example) that might require ditching weight are things which should be solvable by your buddy -- and an attentive, capable buddy is a cornerstone of the GUE system.
The one time you might need to ditch weight is at the surface (if you have lost the boat, for example, and are dealing with a long float). If you are properly weighted, you shouldn't be very negative at all at the surface with an empty tank, and if you are boat diving, you should be carrying a marker buoy that has additional flotation.