Stand alone GPSs are no doubt better, but remembering to bring them with you every time you get in the car is a pain. They also get stolen if you leave them in car (or just a broken window if you don't). Having a good one on your phone helps alleviate that problem. But also, to match the convenience of having it built in, you'd have to get a holder for your phone which makes it just as accessible as the HUD. But I can imagine that some people find a holder like that to be ugly and inconvenient.
I guess you can also remember to bring a bluetooth every time you get in the car, or you can buy a separate one JUST for the car, but it's simply not as convenient as having one built in. Do you make a sacrifice having it built in? Sure, in price and quality.
Some people just prefer convenience.
Diff strokes.