The 12" concentric coil is good for fields that have not been inhabited or just really clean areas. I cannot think of many PROS for the 12" stock coil, I have one and never use it. It would cover more SURFACE ground per sweep than SOME coils, but the shape of the signal from a large DD coil has even better coverage AT DEPTH.(see pic#2) At depth, a concentric coil only covers a tiny point so even if you step just 2" or 3" you could miss a dime at the max depth for the coil. Most lawns and within city limits will have iron and trash that will beckon a different coil so you don't null or mask.
This image shows the signal shape of concentric coils vs DD coils. As you can see, the concentric coil(12" and 950 stock) will have 3 targets in the signal at the same time, making that barber all but impossible to see.
___CONCENTRIC COIL_____________DD COIL____
http://www.ohiometaldetecting.com/uploads/2/4/7/2/2472162/4074187.jpg
This image shows the COVERAGE front to back and demonstrates how thin the signal is side to side of a DD coil.
__Concentric_______DD(front to back)____DD(sideview)
http://www.ohiometaldetecting.com/uploads/2/4/7/2/2472162/5980892.jpg
Another scenario, this is what a lot of parks would look like.....
http://www.ohiometaldetecting.com/uploads/2/4/7/2/2472162/95455.jpg?538x157
Source: http://www.ohiometaldetecting.com/2/post/2009/09/shes-right-you-really-dont-need-an-additional-searchcoil.html