Something else I noticed recently - the way they explain it on the Minelab website it sounds like you even get all the same frequencies when in multi frequency mode, it's just that you can't select the 20/40 to use as single frequencies on the 600. So basically it's the same machine in that sense unless I'm reading it incorrectly. I think you're right i.e. it won't even matter for most people but I think Minelab plays the psychology game on us feature hungry-folk to get us to buy the 800
Everything regarding the frequencies are the same on both machines. It is just the gold/prospecting option that isn't there. With the 800 you get the wireless headphones, a backlight with more than one setting, gold prospecting mode, slightly more options for customising, and an option for saving a group of settings that you use often.
It is pretty much the same machine. Download the manual and you will see near the back of the 60-odd page doc a table with the differences between the two. In my mind, you don't need to get the 800 if you don't want to.