Two major reasons, the first is power draw. Once you go over 4.0Ghz the power draw of many CPUs skyrockets. AMD might brag about having 4.2Ghz Octocores, but those things draw upwards of 160W of power.
For reference, the power draw of a desktop Quad core Ivy Bridge i5 at full tilt is 75W (and it can turbo up to 4.2Ghz), most of the time it sits around 45 - 50W.
The second (which is related to the first) is thermal constraints, higher clock speeds mean more power is needed, more power translates into more heat. On a desktop quad cores can run at 3.8Ghz without breaking a sweat. The same cannot be said of laptops like the MBP, the small enclosure means that same 3.8Ghz quad core would overheat, so down to 2.2Ghz it goes. Dual cores can run at higher clock speeds because they don't generate as much heat per clock compared to quad cores.
Also keep in mind that the CPU can turbo up to higher than advertised speeds for short periods of time before reaching its thermal limit and throttling back to normal
What will determine what works best for you is the type of work the machine will be doing most of the time, and preferences like the screen size.
Unless your workload involves software that is multithreaded and can take advantage of the quad core, a dual core provides better single threaded performance (which is the majority of tasks) due to the higher clock speed that it can run at. Turboboost narrows the gap for single threaded performance between dual and quad cores quite a bit, but its still something to think about.
Of course none of this matters if you don't like the size of the 13' screen.