I think performers are picked for events because they are GREAT at what they do...therefore, they should do what they do. Why lipsync if you're a singer?
so what if you mess up a little, people won't care...probably like you more because you're human like the rest of us.
To me, it's a lazy cop-out...and when you lip-sync, it makes us wonder if anything else was done to it, like a little auto-tune.
Not that I think Beyonce isn't a fabulous singer...she is so do it!