URL bar is the main clue to users for identifing where they are - hiding it will potentially make the experience confusing or even leave a negative impression - i.e. "Why is the website taking over my browser? Is this a fraudulent website that hides the address bar?"
Hacking away operating system or browser defaults via some "clever" technique is hardly ever good usability. @FlorescuAdrian mentioned visual and technical inconsistencies in his reply, but even more sever is the general disregard for the user's preference and browsing environment.
This technique is similar to automatically opening popups, jumping to fullscreen mode or playing loud music without this being based on user interaction - this is a disruptive intrusion into the browsing experience of users and should be avoided.
Just because some designers / developers use this hack to get the badly craved screen real estate, it is by no means a good practice. If the device software producer would have thought it a good idea for any website to impact an part this essential to the browsing experience, there would be native APIs for accessing such a feature.