I was only speaking to what they do. Can't say I've ever had any issues with unused RCA inputs.
... so this is basically worthless then?
RCA jacks have, what, a 2--3mm inner diameter? Only radiation > 100 GHz could sneak in there, and then only with direct line of sight 
I have had first hand experience with circuit design gone wrong because I left an unused AND gate on a 7408 chip floating, which caused undefined behavior, but the answer was to tie the floating inputs to ground (or high, i don't think it mattered, as long it wasn't left to oscillate wildly).
If it weighs the same as a duck, it's made of wood, and therefore....
If I'm missing something, please let me know!
Marantz (the original) used to supply shorting RCA plugs with their preamps for unused inputs. The idea was if you dialed your input selector past an open input there might be a little stray C that could cause a small pop, or the lack of termination might raise the noise floor a little. But the concept was to ground unused contacts on the rotary switch reducing crosstalk between used inputs. I still have one of the plugs, it looks like the connector from a late 1960s Switchcraft molded cable, but without the cable. I don't recall measuring any effect, or this being a real issue, because...well...very few of us listen to unused inputs...but anyone with a Model 7T, and probably earlier, got a bag full of them for free. The way I used the 7T was as a phono preamp with one input and one output, so the selector switch was unnecessary, and I never bothered to test the effect.