Well! Basically it means that he will be missing the potential of that SINGLE EXTRA speaker. I am guessing 6.1 is FRONT L/R, SIDE L/R, REAR L/R, Subwoofer. With a 5.1 reciever or if I consider correctly, a computer that can only take 6 channels: you will not be able to use the side channel speakers. 5.1 is FRONT CENTER/L/R, REAR L/R, Subwoofer.
Its still an exceptional set though. I have a bose 2 channel speakers for 100$ which looks cheap but works almost as good as my logitech 5.1 at home in quality. the sound is exceptional and altogether explainable by only saying "ITS A BOSE". Spectacular.
However, I guess my replies are in-line with using these as speakers for a computer. So probably others advice might be more helpful.