I'm not necessarily opposed to either lip-synching on television, or using tracks. There are legit reasons to do both, although sometimes a lazy artist will use this technique to coast.
What concerns me about the Saturday Night Live incident is that Ashlee shows us that she is not in charge of her own actions. A strong performer could have waved it off, given a "cut" signal and walked right up to the floor manager and gotten the word out to dump out to the right track. A VERY strong performer would even be able to whip out a little improv or a capella on the spot, or simply straight-up announced into the mike something like "whoa, guys! We cued up the wrong backup vocals. What I WANT to sing is the next single..." and vamp while the virtual backing tracks got reset. Worst case: the director dumps to a "technical difficulties" slide while she's resetting things. Had she done either of these, Ashlee could be enjoying the most creatively memorable SNL performance since Elvis Costello's SNL debut.
Should you be nervous on Live TV? Hell, yeah! But to make a living at this stuff you need to be able to roll with the punches.