the C300 is limited by SATA2 and benefits from SATA3... the problem is that SATA3 is obsolete before even having caught on, intel still hasn't integrated it into their southbridge, the OCZ IBIS is already limited by SATA3 (so it doesn't use it, using HSDL instead), and it appears that a good amount of gen3 SSDs (coming in the next month or so) will also max it out or get very close to.
but it can't hurt to have SATA3... the question isn't an arbitrary "is it worth it" (its better than SATA2, so why not), its "how much money is it worth". So when comparing different models you know how much extra you are willing to pay for SATA3.
are you considering a new build and wondering if you should get a SATA3 mobo or one without? or do you already have a build computer and you are thinking of adding SATA3 capability to it? (much more expansive then doing it ahead of time when first building it)