1. Reading an sms on the phone doesn't mark it read on google voice and vice versa (it's like having pop email)
2. If your phone is off and you get a call the phone won't let you know you missed a call like the google voice app would. This is important in NYC where you're often in the subway and want to know if you missed any calls.
There might have been other issues but these were the two that I remembered off the top of my head.