Yup, got where you are coming from, but that's for some parts of the world. Others (like South America and China) have multiple high altitude airfields where both the destination and alternate are high. That's why I didn't just quote from the QRH, I did so with context. Last time I was at MMMX there were multiple A320's operating.... And Jonty has a real point too as the A321 has the same weight limit as the A320 at that altitude.
But I agree, Flap 3 shouldn't be the Norm...