Sure, I know they want brand recognition. I'm sure Pepsi is envious of Coke's culture status. But regardless, with my usage of it the specific brand doesn't even enter into my mind. So I'm not exactly sure what advantage is gained. And I'll always buy the cheap brands of "Q-tips" and "Kleenex".
I think having an initial monopoly on a market is a factor on the most common substitutions.

Oh...and every beverage dispensing machine is also a "coke machine" regardless of what brand is sold.