Except that indicator is totally useless. If you compare Wii Sports and Gears of War sales, all you can tell is that easily accessible E-rated social game involving physical activitiy is more popular than an M-rated shooter. But of course they were created with different goals in mind and the very nature of GoW restricts its popularity. There areofcourse more niche types of entertainment than shooters, but that doesn't make their representatives automatically worse than Gears of War!
If the nature of a game restricts its potential audience then why not consider that nature to be an inherrant flaw? If a game has what is considered to be broken gameplay then that is a flaw because the gameplay is not up to standard. If the way the gameplay is structured is broken for many people then how can that not be a flaw of the same nature if all flaws introduced into games are deliberately put there for any number of reasons. Just because a few can look past the flaws doesn't mean they aren't there.
I think this all comes down to how you are using the term 'quality.' A thing does not need quality to be popular, nor to be of value. Some 1970s Doctor Who episodes severely lacked quality, with bubble-packing and sock-puppet monsters running amok in quarries, but they were excellent entertaiment none-the-less and worth the viewing by millions that they had.
As I understand it, your argument is really, "doesn't succesful sales of a title prove its value regardless of how well rated it is, showing review scores aren't the be-all-and-end-all?" That would be quite a different discussion to this one!
Doesn't a thing need to have desirable qualities to be of any worth? Any given thing has qualities, but they have to be desirable qualities before it can be considered a high quality product. Therefore high quality would really mean that the thing has a lot of highly desirable qualities.
Quality is a measurement of attainment by certain criteria - how well something last, how well it looks (painted inside the lines, lack pof rendering artefacts, high resolution, etc.), how well it does its job,and so forth. Quality is mostly objective, and quality control is an objective part of product development and production. Whether a game is perceived as good/fun or not isn't a measure of quality, and is disparate from quality. Typically higher quality products meet with higher approval, but you don't need quality to gain customer satisfaction or generate appeal.
–noun http://dictionary.reference.com/browse/quality
1.an essential or distinctive characteristic, property, or attribute: the chemical qualities of alcohol.
2.character or nature, as belonging to or distinguishing a thing: the quality of a sound.
3.character with respect to fineness, or grade of excellence: food of poor quality; silks of fine quality.
4.high grade; superiority; excellence:
–adjective
16.of or having superior quality: quality paper.
17.producing or providing products or services of high quality or merit: a quality publisher.
Even so the nature of these definitions are arbitrary. The reason why I wonder about quality being indicitive of sales is that all over forums and internet blogs and articles I see products being disparaged for essentially made up flaws and it doesn't matter whether they are games or software or physical goods like the iPhone or the iPad. Many good qualities can be glossed over or ignored in focus of the percieved bad ones so this is the reason why I wonder if sales are a better indication of how good a product is over and above reviews and feedback from interested/disinterested people on the topic.