I think that people are reading way too much into the percentages.
Even the pure (yellow) gold that most people refer to is a combination of gold and other metals. Pure gold is too soft to be worn for everyday pieces of jewelry.
The percentage of gold in Rubedo is not meant to "scam" people, it is manufactured that way on purpose because the higher percentages of copper (and other alloys), give the metal a warmer tone. The warmer (or rose) tone is exactly what would be considered a "better" color for those of us who prefer a rose gold-colored metal. If Rubedo had more gold and less copper (and other alloys), then the rose color would be very faint and then what would be the point if the intended purpose was to design a pink-colored gold? It's not like Tiffany is tricking people - the content of Rubedo is out there for the world to see.
It was likely designed because of the increasing popularity of rose gold and the high price tag reflects not only the Tiffany name, but also the popularity of rose gold in general. I think it's funny when people stick their nose up at it because it doesn't contain "enough" gold. It wasn't designed with the intent that it is an inferior product - it was designed as a warmer-toned gold color for those customers who desire rose gold. Obviously, if you don't care for rose gold, then you wouldn't be purchasing Rubedo anyways....