Every book should have one. Even if it is not the best. It should at least have something based off the book. If it is about Paris, do not put the CN tower on it.
I have not really seen book covers like that, but there probably are some. Another thing, don't add too much on your book over. You do not want it to be too complicated, one or two things on it is simple.
People's choice in font is not very good to be honest. They have like, 6 different fonts on one cover. 3 should be the max. And colouring it? I personally do not like coloured font on book covers, unless they are like, dark, dull colours. Not bright or neon.
