I can manually switch when I hear the voice. If I wasn't looking at the monitor then I'd need the voice to get my attention anyway... it only saves a couple of clicks (only one with a good skin). If I (or a passenger) were using the screen, I wouldn't want to be interrupted, and I could just use the audio directions while continuing what I was doing.
I definitely want to be able to put a next-turn label in the top title bar though. I hope an iGMon for iG 3.0 is released soon.