It's the old dpi/ppi thing again
Yes , agree there are a number of confusions about that
In fact relation between printer output image pixel resolution (PPI_print) and printer Dots Per Inch (DPI) resolution could be expressed in general (not going deep into details) as follows:
PPI_print = Function (DPP, DPI, Paper_Quality)
DPP (Required Dots Per Pixel) = Printer_Driver_Mapping_Function(Required_Pixel_quality, Number_of_inks_in_Printer, Avaiability_of_Variable_Droplet Technology, etc. )
Modern printers (e.g. Epson Stylus Pro 3880 or 4900 and others ) can provide final print resolution above 720 pixel per inch (required output PPI is set in print settings dialog box in imaging editing program).
Here is discussion of printer PPI ( not to be confused with DPI) - printed images final resolution using HP 932c http://forums.adobe.com/message/4266349
"I could see significant reduction in the finest details in the 300 ppi print vs. the three higher resolution prints, and a slight reduction in the 567 ppi vs. 720."
So the argument that using Canon EF24-70 F/2.8M2 on a7R does not give any benefit for printing compared to Canon EF24-70 F/2.8M2 on Canon 1Dx is not correct.
Though difference would be almost not noticeable when downscaling a7R file to 1Dx size (details are lost in downscaling) but when full-res a7R image would be printed on A4 or A3 then difference will be visible especially on A3 print which could not be considered large - if you want to put picture on the wall in your room the best minimum size would be A2
. Also higher quality higher res image gives more flexibility to cropping.