The promoters of the current Britney Spears world tour have been pretty careful in their descriptions. It's not so much that they advertise it as a Britney Spears concert, of popathon, it's rather that it's a show called "Circus", which stars her.
They can thus rather cut thruogh all of the questions about whether she's actually singing live or not. Which, of course, she is most unlikely to be doing.
It's not just that her voice has never been that strong, rather than Britney Spears has always been a dancer rather than a singer. And so all of her efforst are indeed put into the dancing, not the singing.
And if we're to be quite honest about it, no one can in fact gyrate around a stage in quite that style and still be singing their heart out: no, there's often a great deal of help from something I mae in hte studio before I set out in these shows.