Yeah, at least Bob Dylan still gives us the real deal. Say what you want about his voice...
In sort of a semi-deserved fairness to Britney, if she knows that her voice is not being projected to the audience, chances are she's just humming along to make sure her lips are in sync. Thus, she isn't really trying to sing so much as lip sync. Notice that she skips parts and says other irrelevant stuff when her back is turned to the main audience, which probably indicates that she's confident that her back will be on camera during those segments.
What I'm getting at is that what we're hearing isn't intended to be her "best" singing voice because she's mostly focused on getting her lips synced and of course the choreography. Right before she speaks to the audience, she's hearing the producer say, "3-2-1, you're on" in her IEMs. So the rest of it isn't really the best way to judge her voice.
Although saying that, it's still pretty scary! Especially those parts where she actually seems to be singing her heart out.