That isnt completely true. I have an early 2011 15 MBP and it has SATA 3 6Gbps on BOTH the HDD and ODD ports.
It has been well established that the higher end MacBook Pro's has issues with 6G, even though it reports that it is 6G.
Check this link and scroll to the bottom. Diglloyd covers this in depth in older posts.
http://macperformanceguide.com/blog/2011/20110915_1-MacBookProEFI.html