Do you care about the opinion expressed in this newspaper's theatre column? If you do, there is a growing body of opinion that says you are in a shrinking minority. Social media is where opinion increasingly counts.

It has got so that some producers pluck praise from a passing tweet and plaster it on their production's poster as if it were the considered view of a discerning and informed mind.

Which, of course, it might well be. But how do we know?

The line that ring-fences reviews can also be blurred when the arts become news.

The two biggest events in the arts world this year were such hot news, newspapers bent over backwards not just to report them, but to be first to give their readers a critical response.

The first - touted as the literary event of the century - was Harper Lee's Go Set a Watchman, the sequel to To Kill a Mockingbird.

Reviews were embargoed until July 14. On this side of the Atlantic at least, courier bikes were primed to race copies out to select book reviewers who had but a few hours to read the 288-page volume and write a review.

In the event, the New York Times torpedoed the plan by publishing early, leaving other newspapers scurrying in its wake to rush out a critical response.

But, apart from a few newspaper and publisher noses being put of joint, no harm was done. No controversy ensued.

Which brings us to the other major arts event that made the news this year. This time, controversy certainly ensued.

More was written about this play before the official opening night than any other theatre production in recent history. We are talking about Benedict Cumberbatch's Hamlet, or course.

During the show's unusually long three weeks of preview performances - a period used by directors and actors to hone their production into shape before the critics turn up - the papers frothed with Cumbermania, about the fans that queued at night; the performance that had to be restarted because fans knocked their idol off his stride by filming him with smartphones; Cumberbatch's desperate stage door plea to put the phones away.

And then, just when it seemed everything that could ever be written about a play before its official opening had been written, the press were the subject of their own headlines when, in an unprecedented move, three national newspapers published a review after just one preview performance. But none by regular critics.

The arguments over the rights and wrongs of breaking the embargo are probably theatre's equivalent of the Westminster bubble: fascinating to everyone in the business of putting on shows or writing about them; utterly inconsequential to everyone else.

Still, here are two: producers and publicists stoke up all the hysteria they can over star-vehicle shows, so it's no surprise, and a little reassuring, that they can't control their own monster.

On the other hand, judging a piece of art before the artist has finished it is about as useful to a newspaper's readers as reading Harper Lee's latest novel before she has completed it. Yet a searching question has been thrown up by all the fuss.

If theatre critics can behave like bloggers and tweeters, why have theatre critics at all? Why not just allow a stream of social media to provide a constant commentary on a production during previews and then also during the performances proper? Would the public be any less well informed?

For me, the most persuasive argument for the continuing existence of the critic is related to the fact that the critic simply has to go to the theatre.

That is, unlike most theatregoers, the critic does not generally watch a show because he or she might like it; unlike your average blogger, the critic does not only watch productions populated by stars that excite them.

The critics see everything - or everything their diaries allow - whether they want to or not. It's this that allows for the possibility of discovery. There are few things more life-affirming for a critic than dragging him- or herself to a show whose star he or she despises only to leave it with a newfound respect.

There are few things more invigorating than having a nurtured prejudice against a play confounded by a brilliant production. And who else is going to shlep to a pub in some suburb on the opposite side of town to see a play written by a fledging writer about a subject of no particular interest populated by actors no one has ever heard of?

I was once talking to a famous actor (Maureen Lipman) about a certain critic (can't say). "She writes like a fan," said Lipman damningly.

The comment has always stuck with me. It not only highlights the way in which a critic can become useless, but how a critic can be useful. Embedded in that criticism of a critic is recognition that informed, objective opinion about the theatre is as important to those who create a play as it is to those who go to it.

Especially so when some people are happy to get their theatre from a blogger's review of a preview.