Yes, hitting enter in book view is useless. You have to use proper styles in code view. Hitting the space bar a couple of times and then enter will probably result in
<p class="whatever"> </p> which is not a good idea either.
An interesting addition to Sigil would be the ability to specify the behaviour when the enter key is pressed in book view. Options could be: (1) Add class xxx to next paragraph (2) Add class xxx to previous paragraph (3) Add empty paragraph (4) Add br tag.