This becomes especially apparent when you're in a country with a different language and alphabet (Greek, Cyrillic, Chinese, etc). Even if the device successfully pronounces it, you can't look for it on street signs.
I think car navigation and looking at the map are different ux activities though. When you're navigating a car, the street names may not be useful, but when you're walking with a map you can certainly compare what's on your display and what's on the sign (whether it's Latin, Greek, or in hieroglyphs) without knowing how to pronounce it.