"but if a dual takes up significantly less die real estate than 2 processors"
It won't. The memory controller and HTT links amount to only a small percentage of the die. And the larger the die, the more likely the defect density is going to affect a given chip. Still, dual core has a lot of system level efficiencies that are hard to ignore. For example, you can get a single socket motherboard for under $150, a dual socket motherboard for under $300 and a quad motherboard runs you $1600. Those instantly become dual, quad and 8-way with dual core. Being able to use off the shelf, commodity power supplies and cases make a big difference for dual and quad systems using dual core.