Not necessarily true, because sometimes you have to. I used to be a dual UK/US national. I had to be in the UK on my UK passport - illegal not to (although not generally particularly well enforced) and I had to be in the US on my US passport. So, I had to switch passports mid-trip. There are many people in much the same position. And, as anyone with even a small grasp of current geo-politics will be aware, immigration control is becoming massively tighter.