Authors that dismiss requests to appear on blogs outside their genre must not understand how eclectic readers are. If you look at any true book lovers bookcase you will find a wide and vast array of genres. On just one of my bookcases I have steampunk, erotica, YA, cozy mysteries, paranormal romance and urban fantasy in addition to a varied collection of non-fiction books.
You just can't predict who will love your book and who won't. The goal is to get it out there in front of as many readers as possible. Especially to book bloggers who can get your book in front of a new audience.
For one if the blogger requested the author to be a guest the blogger themselves must already be a fan of the author or be interested in the book- which can lead to positive reviews and promotions. If the author dismisses that blogger- they may lose a fan and lose an entire previously untapped fan base because if the blogger is interested in that book even though their blog does not focus on that particular genre- chances are that blogs' readers are an eclectic mix as well.
To clarify I'm referring to blogs that are not completely in another realm as compared to your book - ie any romance book could appear on any romance genre blog, paranormal books can appear on horror blogs, horror books on paranormal and urban fantasy, sci fi on fantasy and paranormal and vice versa but no I wouldn't want to put an erotica book on a site that focuses on YA or children's books or put a LGTB on a site that features Christian books. I'm still looking to keep things related even if it's third cousins twice removed related
The second reason not to dismiss non-genre blogs is that you should always be looking to expand your reach. If you stay in your little bubble you'll never attract a wider audience.
And the third reason- why would you pass up free promotion?