However may not necessarily be true on day of travel when airlines operate out of multiple terminals, which comes back to the "subject to change" issue. And if you told your taxi driver terminal 1 but your flights in terminal 2, either your taxi driver is now at the wrong spot and needs to move, or you're lugging yourself to the other terminal. Fortunately at ORD that's not too big of a deal (the walk from B to E/F in Terminal 2 (and vice-versa) isn't too much worse then the walk from B to C, it's all connected airside, and the baggage system is smart enough to route checked bags to the correct terminal, regardless of which one you checked in to. But that story isn't necessarily the same at other airports, and if the customer relies solely on what they received at the time of booking (or via GDS) then they could end up in the wrong place when they actually travel. The current UA setup at least makes the customer look to see what terminal their flight is actually flying out of on the day of travel, and presumably reduces instances of customers going to the wrong place.