Apparently, Art is still confused and thinks Twincy is a cool way to talk about Minneapolis/St. Paul. He is wrong. His reasons may seem sound but I'm going to break them in two over my leg like Bo Jackson breaking a baseball bat.
Equity: Art says msp is improper because it gives St. Paul an extra character. Well boo hoo, last time I looked St. Paul is the capital city, it deserves its place in the sun.
Specificity: It is true that MSP is the airport designation for the Minneapolis/St. Paul International Airport. However, this is an easy fix. When talking about the Twin Cities just use lower case letters. It makes more sense to use an airport designation so outsiders know what we are referring to. As someone from Jacksonville, what MSP stands for and they will most likely know. Ask what Twincy means and they will probably think you are talking about an obscure sex act.
Uniqueness: This is another poor argument. MSP is widely used, it's used by Amtrak and every airline in the world. Out of all the cities in the world, only the Minneapolis/St. Paul area can use the MSP designation. Twincy is unique only because it's stupid and no one wants it.