There is another important reason to ignore intent when examining a work of art.
Sometimes, an artist fails.
The artist may have an intention with a work of art, but could fail to achieve that intention. That doesn't make the work unworthy of analysis; indeed, the artist's intent could fail and BECAUSE OF THAT FAILURE, the result could be a great work of art. Regardless of the success or failure of the intent, we still have the results to examine.
And as psychoanalytic thinkers and critics have clearly suggested, human action and intent does not always operate on a conscious level.
Mary Shelley may have intended Dr. Frankenstein to be a sympathetic character. If this is the case, she failed: he is a self-pitying coward who constantly evades responsibility (despite protestations of guilt and blame, he avoids actually being held to blame for anything; furthermore, he takes blame for making the monster, not for abandoning the monster the moment of creation). Does this mean I have to think of Frankenstein as sympathetic because Shelley wanted him to be? Of course not. Does this mean I have to talk of Frankenstein as a failure? No, I don't, and even if I did, it might not change the merit of the work itself: regardless of what Shelley was trying to do, I must deal with the results on the page.