While most relatively current mobos default to 1333, 1600/9 is basically considered entry level DRAM and has been for over a year now even on AMD which still has numerous mobos that default to 1066 (a result of them having weaker MCs (memory controllers)...Intel put out the XMP standard (which starts at 1600) to make it easier for people to set up their their faster DRAM as they realized (going back to DDR2) that people wanted faster sticks....
The biggest thing about DRAM is that few people understand the workings of it or how to set it up (if need be) manually, there's not a lot of people that even want to mess with the base timings (i.e. the 9-9-9-24), and far fewer that will touch secondary timings (let alone tertiary timings). I've had occasion to deal with techs from Corsair, Mushkin and others that have no idea what to do with secondary timings like tRFC, tFAW, etc especially to get mixed sets to work, which can be important, i.e. many people will buy 2 sets of 2 sticks rather than a tested 4 stick set (to save a few dollars), and then don't understand why they won't play nice, they don't realize that the XMP programming is by the packaged set and for example a 2x8GB set might require a tRFC of say 220 the same exact sticks in a 4 stick set might need a tRFC of 255-278 to run smoothly