Would you make this same point if the two were arguing over whether or not the moon is made of green cheese? After all, neither of them has ever been there.

Or would you accept that, since there's a colossal amount of accumulated, interlocking evidence in favour of the moon being made of rocks 'n' stuff, the burden of proof is upon the greencheeseite to justify his claim?

I think it's Ken Ham who produces the fallacious "Were you there?" gibe to counter those who criticize his claim that there were dinosaurs on Noah's Ark. The fallacy lies in the (false) assumption that the two sides have equal evidential merit: I'm sure Ham would agree with me that it'd be ridiculous to suggest Noah took boxfuls of Barbie dolls aboard the Ark, yet his "Were you there?" rhetorical tomfoolery is equally valid in that instance.