Are subjects important? Will incorrect subjects on an ISBN have a big impact on its distribution? This is a very broad question that can either be answered with a word ("yes") or a dissertation. We will try for something in between.
Just as providing correct title, author, publisher, and pricing information is important, correct subjects are important if you want your book to be properly represented in the marketplace. They are an integral part of providing well-formed metadata about an ISBN. Leaving book discovery to keyword searching and title scanning alone is not enough because keywords often retrieve "false positives" and because the subject of a book is not always self-evident from the title: "Orange is the New Black" is not about fruit or color theory, and "The Glass Castle" is not about architecture.
Further, many unrelated items come back from a general keyword search, such as "art". Examples include: an "Etch A Sketch" produced by Ohio Art, "The Art of Racing in the Rain", a novel, and "Artemis Fowl", a character in a children's book series). While I am not able to assign a numeric value to subjects, or rank the various fields in order of importance, at the most basic level having a correct subject on a book means that it will appear within a list of similar titles that are accessed when someone uses subjects as a search point (and this is a very common way for people to look for books).
Applying the subject code requires you also apply the most relevant code for the item. If someone wanting a book on art uses a link from the subject list for "art", or does a subject search on the term "art", only books with the subject code art will appear in the results. If someone is looking for "theater" books, and sees a book that they feel is unrelated, such as one about Michelangelo, they will probably just skip over the book. And, of course, the misclassified book will not appear when someone is looking at all the other "art" books.
Websites also often use subject codes to recommend books to customers. This automated process will not pick up a title for recommendations if it is lacking subject codes.
In addition, assigning the subject "art" puts a book in a general classification. On the most basic level, the object of subject classification is to assign the most specific, non-redundant subject(s) appropriate from the subject heading list you are using. So if you are using BISAC subjects, a book on Michelangelo could be assigned "ART016000 ART / Individual Artists / General" and "ART015030 ART / European" and "ART015080 ART / History / Renaissance". Depending on the focus of the book, it may also be assigned "ARC005040 ARCHITECTURE / History / Renaissance", or "ART026000 ART / Sculpture & Installation" or "BIO001000 BIOGRAPHY & AUTOBIOGRAPHY / Artists, Architects, Photographers". If you were using Library of Congress subjects and the book was about Michelangelo's design for the Sistine Chapel, you would "Michelangelo Buonarroti, 1475-1564" and "Cappella Sistina (Vatican Palace, Vatican City)".
In summary, applying correct classifications to an ISBN as part of its metadata is very important - and the more detailed the subjects are, the better.