There are a lot of books that have been written on a form... following the patterns and expectations of a recognized story type. Writers take the form, the expectations, mix and match norms, and bend a story around the conventions. Some of these can be very entertaining, good examples of the genre.

There are also novels that are original, delve into issues that are immediately identifiable across a broad group of readers, are well constructed, lead to additional consideration of a reader's knowledge/reality. These novels can fit very comfortably inside a genre classification. They also can be held up to a high standard quality, or termed literature.

Does calling SOLARIS a science fiction novel demean the value of the story? No. Does the story fit with the basic definition of the genre? Absolutely. Why does it matter then? Well, appropriately enough in a discussion of this novel, it only matters in the minds of the people who process the label, and the prejudices they bring.

I am not a big fan of labels. I also don't chase after books to experience the same set of expected conventions. I pick up a book that looks or sounds interesting or entertaining, and I read it. It could be a history, a memoir, a Classic, a mystery, or even sci-fi & fantasy. Heck, I just finished reading a serial gothic story, the Harry Dresden novels, and enjoyed the heck out of them. For some people, they really enjoying knowing what they will be getting when they pick up a novel. Genre distinctions, especially sub-genres, are really appreciated by a whole lot of people. I really don't find anything the matter with that.

The only thing that I trip over is the idea that by calling a work like Solaris a science fiction novel, that you are somehow devaluing the novel itself.