For pure art it may not matter (though I suspect that wealthy artists have adapted to market demands, but won't admit it), but for functional products it matters a lot.
This is sort of an amusing debate, because usually I'm the one pushing to make things more different and unconventional. However, to make changes really take root, you often have to "boil the frog".
I think for art you can look at a work in 2 ways: the subject and the style. The subject can and usually is highly influenced by the buyer, but the style of the artist, which I argue should be a manifestation of the artist's core values, should not be influenced. If it were, the artist would lack core values and consistency - which is never the case (that I know of) for meaningful artists. So again, the buyer chooses the artist for their style, and then goes from there on possibly suggesting a subject (such as a portrait), not the other way around.
You can easily argue that Apple is taking this same approach to technology. They present a distinct style of products - usually highly designed/integrated and simple to use (some would argue almost inspiring to use) in their many different products, but the type of product they make is highly influenced by what people want. For many, Apple products might be too simple or too expensive, but there is a growing group of people willing to pay extra for Apple's style of technology.
Now, if Apple were to change their style based on market research for every product, that would be the death of the apple brand as we know it, and they would morph into yet another cloning company. The familiarity idea you were talking about also breaks down with the Apple example, as who knew they wanted a Mac before it existed? It certainly wasn't familiar, nor was the Iphone, etc. But that issue is another topic. My point was that to be a successful artist adherence to a core set of values is key (in the case of business you could call it branding). This issue shows up alot in music, where people will find that a band has "sold out" by changing their unique sound to appeal more to a mass audience thus losing those original fans.
The point here is that for an artist to make authentic art, the work has to be a reflection of that artist's core values regardless of what they are. It is that core that gives consistency to an artists work, and it is being true to that core that is necessary (but not sufficient) to make meaningful art.
Rand's point was not at all that Roark would "force" his work on his clients for whom Roark could care less, but rather that the client had to choose Roark because Roark's work resonated with the values of the client in the same way that someone who buys art is hopefully doing so because that art resonates with their values.
The point Rand was making was that culturally the kind of art Roark was producing, in the beginning of the book, is not popular and that only a few desired it. By the end of the book, Roark had found success in finding a growing body of clients who wanted his style of design. He had found his niche and had remained true to his core values as an artist - integrity being the primary one.