I think they just put 6.1 inputs rather than just 5.1 becuase it don't cost hardly no more and if there was a format to come out later with 6.1 then peiople can still use it.
Actually I can see the point of full 7.1 inputs. Especially if you're using dipole surrounds but direct rears. SACD / DVD-A may sound better with the direct rears so connecting to left / centre / right / (leave surround left right empty) / rear left / rear right may give you best sound.
Good point though I guess that wasn't the idea behind it.
6.1 will probably be found on amps with 6 internal amplifier stages while 7.1 is rather common with 7-channel amps or 5-channel amps with 7.1 output like the Denon AVC-10SE so that you can control the volume of all 7 while using an external 2-channel power amp for the surround backs.
Of course Denon did a clever thing and allows re-routing of the power amps so that the 2-channel amp mentioned above could be used for the fronts.