It’s hard to get an accurate sense of it, because it’s very uncool to admit you consider points and critical reviews in buying wine, and it’s de rigueure in wine discussions to do the rant about score whores and Parkerized pancake syrup and so forth. So, predictably, many people just don’t admit it, or substantially downplay the extent to which they do it, in discussions on bulletin boards and at wine events and gatherings.
It would be like asking people how many of them wash their hands after using the restroom – presumably 100% will say yes, and yet we all know that a high percentage of people do not, and an even higher percentage do not when nobody else is around.
I don’t know the answer to the question, but I think it depends a lot on (1) what sector of wines and buyers you are referring to (e.g. all wine consumers vs. consumers who buy premium wines vs. true wine geek/collector types), (2) what categories of wine (BDX vs. Napa vs. Burgundy vs. Australia etc.), and (3) how outlying the points are (the public responds quite a bit to outlying high scores, and to the 90 point threshhold, but not very much to the difference between 91 and 93, or 86 and 89).
If you’re talking about all wine purchased in the US, it’s a very low percentage, because most wine is cheap bulk stuff that doesn’t even get rated and wouldn’t get good scores if it were, but I would guess you’re more interested in “premium” types of wines, where the percentage is definitely higher. Particularly when you count the “halo effect” of scores over time which Roy Piper described – you may not buy a given vintage or bottle of Harlan, Kosta Browne, Pontet Canet, Carlisle, Montelena, etc. because of scores, or even knowing the score for that particular bottle, but a sustained period of high scores from critics may still have played an indirect role in your hearing of the wines, going out and trying them, forming a favorable impression of them, becoming a regular buyer, etc.
One other thing to note is that many people who don’t follow WS or Parker, including many who are the loudest about their disdain for scores, view other publications and sites that offer scores of some sort – Cellartracker, Tanzer, Pinot Report, Gilman, user reviews on eBob and WB, blogs, etc. So, there are scores and then there are scores . . .