I was just in the local bookstore yesterday thinking about this same topic. I am of the belief that eBooks should NOT cost as much as the physical book, mainly because they require no materials, and their distribution cost is MUCH cheaper. It can't possibly cost over 50% of the cost of a book to produce an eBook!
But yes, I did then think about the portability and ease-of-reading factor (I HATE having to try and hold a fat book open with two hands and STILL have difficulty reading in the middle towards the spine).
For photo-heavy books and cookbooks I agree - I definitely want the physical copy. I tried once to use a recipe on my iDevice when baking, but it was not worth the mess!
And for the record: I have seldom bought a book at cover price, if ever. I am also a fan of the local charity/thrift stores and get most of my cookbooks, etc from there. For the rest, I often wait until they come on sale on Amazon.