Most modern engines thread well and actually can be very CPU heavy,ie,try using a dual core against a quad core in a MP scenario in some games and see where it leaves you.
OTH,any PC game released in 2012 which can barely use two cores is indicative of two things:
1.)Cost cutting during development.
2.)Very poorly optimisation, indicative of cost cutting at a management level,especially if there is a console version(a console version would have to use multiple threads well).
Basically most of your CPU is idle when playing the game which means most of the CPU is not being pushed. If you don't overclock(the vast majority of PC gamers),going from a Core i3 to a Core i5 does not yield much improvements,which is pathetic.This means additional costs pushed onto the consumer,or worse gameplay. Such companies are holding back PC game development.
Older games which have been around for years,might be understandable,but not ANY game released in 2012.
All the modern engines tend to use multiple threads effective - Frostbite 2,CryEngine 2,id Tech 5 and the forthcoming Source2 and UE4. Engines such as Frostbite 1.5,later versions of the UE3 engine and Source all use multiple threads effectively. In fact most engines which can be used across multiple platforms now thread reasonably well.