Both the core 2 duo and the core 2 quad processor lines are excellent in thier own rights.
The core 2 duo processors offer far more flexibility for overclocking than the quads do, which is one reasons why many gamers prefer them over the quad cores.
Additionally, the lower heat output of the duos makes them an ideal choice for a powerful home theatre pc where there isn't much room for additional cooling, and silence is at a premium.
The quad cores offer a lot of sheer power, but little flexibility, and even at stock speeds can be challenging to keep cool because they put out so much heat.
Currently, both processor types have their applications, and because of this neither one is really "better" than the other.
Eventually a revised quad core may emerge that offers the same appealing features that the core 2 duos offer currently in addition to being the powerhouse which the quad cores are, in which case the market would gravitate towards this all-in-one package. For now though, this isn't an available option, and it's likely that the market will remain split because of it.
Just my .02.