My collection isn't as large as many members here, but still some of the cookbooks on my Bookshelf are Kindle books. They are not taking up physical space in my house, but I can still include them in my searches. I would probaly never use a Kindle cookbook if not for EYB -- I don't ever think to "thumb" through a Kindle cookbook, but I sometimes purchase a title that's a good deal and it's nice to know I can access those recipes when they pop up in an EYB search.
Also, I remember at least a person or two mentioning in the past that they add books to their EYB shelf they know they can borrow from their local library (and I'd assume they Bookmark them accordingly to prevent confusion about what is in their house vs. another location).
It could also be useful to add books to your Bookshelf to "test" a cookbook before buying -- once again, if clearly marked that it's not a book you actually own, it could be helpful to see if a book you are considering ever turns up any results as you go about using the site as you usually do.
If you subscribe to an monthly ebook service such as Kindle Unlimited or Scribd, you certainly could add cookbook titles available through those services as well.
I also imagine you *could* add books to your Bookshelf you know you could access from friends or family if you wanted to, it all depends on how you want to set things up.