You're quite obviously dancing around what I actually said, though. If DPI is "just a marketing number", explain how in your mind there's no difference between printing an image at 100dpi versus printing an image at 300dpi.
"Up to a point, printers with higher DPI produce clearer and more detailed output. A printer does not necessarily have a single DPI measurement; it is dependent on print mode, which is usually influenced by driver settings. The range of DPI supported by a printer is most dependent on the print head technology it uses. A laser printer applies toner through a controlled electrostatic charge, and may be in the range of 600 to 1800 DPI."
http://en.wikipedia.org/wiki/Dots_per_inch
This wikipedia link may clarify what dpi is.
I understand that you are trying to justify size with quality, but that relates more to ppi, which comes from an input, such as a camera or scanner.
Obviously, the higher the ppi during input, then the better the image quality will be when blown up to a larger size.
However, dpi is related to output, such as printing toner on paper. I don't know of any professionals outputting on anything less than 1,200 dpi.
Technologies in printing have come to a point where we aren't really moving past 1,200 dpi, as most human eyes can't notice above that value. Instead, we are now focused on creating more precise lasers that output within that dpi value and use better technologies that can disperse toner more smoothly within that given range.
So, crap in is crap out. You must have high ppi in order to get quality dpi.
As far as the "marketing" numbers of dpi that I am referring to, there are so many other variables that come into play when judging quality.
For example, take a look at a 1,200 dpi print image off a Canon C7000VP versus a 1,200 dpi print image off a Canon iR C5185. They simply do not compare, and this is due to technological advancements in toner, fusing, and many other variables.
I know this is slightly off topic from what the OP initially asked, but other forum posters did refer to printing.
Please understand that there is a difference in terminology between dpi and ppi (or spi).