(WSJ) - PICTURE THE SCENE: It’s early September and the day is drawing to a close, but there is still enough summer in the air to enjoy supper outside. In front of you is a plate of juicy prawns, sizzling away in a garlic sauce. Next to it is a glass of chilled white wine, nothing particularly expensive, nothing particularly special, but at that moment when you reach for a first sip and swirl it around your mouth it is as good as anything you have ever tasted. If I asked you there and then to score it out of 100, you would probably give it full marks.

Therein lies the difficulty of rating wines. While there are perfect situations for specific wines, can you ever really say a wine is perfect? After all, a wine’s taste and character changes almost daily, and whatever any critic says, taste is subjective.

Among wine critics, there is an almost constant debate about scoring wines. Does it help the consumer, or does it confuse matters? Are we dumbing wine down or demystifying it?

Scorers of wine argue that it is a helpful guide for consumers, a shorthand to identifying quality. The opposing camp argues that there is a lack of uniformity, and scores are given out of context. Some ask if it’s a pointless exercise—isn’t a description sufficient?

I can’t help feeling that at the heart of this debate is an element of self-justification. As Caspar Bowes, a British wine merchant with a self-described “visceral hatred of scoring wine,” says, “it is people pretending that they are doing something scientific when they are not.”

There are several different systems for scoring wine. The most famous is the 100-point scale, widely credited to Robert Parker, which is used by the Wine Spectator and Wine Enthusiast magazines. This system looks at the color, bouquet and taste. Each wine is awarded a base score of 50 for being created in the first place. On top of this, points are added—up to five for the color, 15 for the bouquet, and 20 for the palate and texture, and another 10 for the overall quality and potential for development. You could argue it is actually a 15-point scale, as so few wines score under the 85 mark.

The second most common scoring mechanism is the 20-point scale used by the University of California, Davis, recognized as the finest wine university outside France, and several British critics. There is also a five-star scale adopted by Decanter magazine, and a three-star scale, or three wineglasses, the “Tre Bicchieri” used by Italy’s premier wine guide, the Gambero Rosso.

So what is the point of scoring? Robert Parker told me that he began doing it as a reaction to too many wine merchants hedging their bets. “I just thought by using the 100-point scale it is a stake in the ground, the ultimate accountability,” he said when I spoke with him last year.