Flops are to theater critics, as diseases are to doctors. Good health, like good musicals, are never as fascinating as illnesses and failures. Like some doctors, critics may also like to play god.
Take for example, Frank Rich, former New York Times theater critic. He is reputed to be able to determine the fate of a show by a stroke of his pen. A bad review from Frank could kill a show.
This is denied by Ben Brantley, chief theater critic of the New York Times. In an internet radio interview, he defended the critic, saying that if a show flopped it was because it was no good, and had nothing to do with the critic. The latter is merely a messenger, and it would be wrong to shoot him.
What is the role of a critic? I think a critic should help you understand and gain deeper insights into an artist's creation. But most critics fail to do that. They are merely the messengers. They give superficial answers to your question: "Is this show worth seeing?"
Most critics get the answer "right". They correctly predict which shows are hits and which are flops. We only remember the ones that get it "wrong". Indeed, if you read the London critics panning Les Miserables, or Frank Rich's slightly unfavorable review of Cats, you might have missed out on these two musicals. Fortunately for these musicals, positive word-of-mouth was sufficiently robust to overturn the critics' negativity.
In the 4 August 2005 issue of The Guardian, Lyn Gardner wrote about a new musical, Behind the Iron Mask, saying that the "sheer ineptitude of the evening bears all the hallmarks of the West End equivalent of vanity publishing... it is a calamity project." The Evening Standard said that the musical bore the brunt of "one of the most ferocious critical onslaughts in recent West End history". Some have voted it as one of the worst musicals to be performed in the West End.