Perhaps if Disne made credit card deposits required to book online I could see it. Otherwise there is great potenital for problems/abuse of an online dining system.
Folks would book any and all CRT or other popular seating then cancel last mnute. Now, you could say that the system could be designed to eliinate more than one ADR at a given time, but then you have problems with large groups who may want to split up and eat at different restaurants.
As quickly as availability changes at WDW dining, I really see problems trying to do a live online system, and making it extremely user friendly (AKA idiot proof) or protecting everyone from the ADR multi bookers.