Rule #3: Dr. Pepper is not the same as Root Beer.
My default drink of choice is a Dr. Pepper. Oftentimes, I'm at a restaurant and when I ask for a Dr. Pepper, they'll respond with "No, but we have Mr. Pibb." That's acceptable. That makes sense. There are several cross-brand sodas that are a logical substitute Sprite and 7-Up. Pepsi and Coke. Another bad "substitute" is when you ask for Sprite, and they offer Mountain Dew.
However, Dr. Pepper and Root Beer are nothing alike. Nothing. Other than the fact that they are a brown soda. But so is Pepsi. Root Beer is not an acceptable replacement for Dr. Pepper.
Don't get me wrong - I actually like Root Beer! I just don't like it when Root Beer is offered as comparable to Dr. Pepper.
Special Note 1: Actually, it is kind of entertaining that (I think) Coke and Pepsi don't actually taste that much alike, yet they are considered the "other" choice. I don't know anyone that is an "only 7-Up, no Sprite" drinker, but most people are either dedicated to either Coke or Pepsi.
Special Note 2: I don't ever drink Coke or Diet Coke at all, but I'll take Pepsi on occasion. I will randomly drink Diet Coke with No Caffeine, though. But only out of a can. And only after it's been refrigerated.