1. ADC allows you to connect a monitior with one cable... Power and Display cables are contained together in one cord.
2. ADC is a Digtial Signal, and since the output is never converted to analog, the actual image is crisper and truer.
That said, when I bought a new monitor for my Mac... I got a CRT LaCie 19" that connects using VGA. Best balance of screeen resolution, quality and cost, IMHO.