The spine is the most undervalued aspect of book design. This is especially true today, when so many indie-books only publish electronically. Writers are doing themselves a disservice by restricting the market for their books. Granted, the majority of my sales are eBooks, but I also sell a respectable number of print books. I do well with libraries, and despite not pursuing bookstores, I've found my novels carried in the big chains and independent stores. Additionally, some readers who prefer electronic books still look to see if there is a print version to strengthen their purchase decision. My recommendation is that authors insist on a printed version.
If a print format is produced, then a book becomes a three-dimensional object and must have a back cover and spine. The spine is the most important.
Unless you’re a bestselling author, your books will not be stacked on tables at the front of the store, so only the spine will be seen when your book is on a shelf. The same is true for a library. If you want to sell your next book to a library, people must find and check-out your previous books. So pay attention to the spine. Getting sales is tough enough without your book disappearing in a maze of other books screaming for attention.