Art critics can help others know if getting that piece of art work would be something they would enjoy or not. We all have different tastes so it is important to find a critic with a like-minded view on art as you. Overall though I do believe they are relevant, because they help people get an understanding for that artist or art work ahead of time.
I think that art critics are relevant in the same way that the lay person's opinion about art is relevant. Art critics offer another opinion, and supposedly theirs is an informed opinion, and it gives the beholder of the art another perspective to ponder. If a person does not agree with a critic, it doesn't mean they don't understand art, though.
That is a fact: the quality and beauty of any art (painting, song, movie) is only a question of point of view, what is beautiful for someone is not always so for another. It is all subjective. I can understand that they make people understand better an art, however, they should stop trying to give a positive or negative review of what they analyse. We should just call them art analyzers instead of critics and bring a lot of reforms into that job in order to make them respected.
To appreciate art is not left to one individual but the population at large and pieces of art should be judged by the populace rather than specific people trained in the field. Art is left open for interpretation to all and one critic's opinion should not validate or invalidate a piece of art.
Relevant, to me, would imply that art critics are useful to their fields. It seems like the vast majority of critics have agendas that go beyond, or even exclude, meaningfully judging art. A relevant critic, much like any reporter, should be able to step aside from their personal preferences and biases and judge works neutrally. Sadly, most of them can't see beyond their personal borders and are snidely disrespectful of any who don't agree. I class most art critics with the journalists, reporters and other press people who say they are reporting information to the public but in fact they are just looking for attention.