What's the chief advantage of Memoir over "standard Book" for a novel

memoir is better for this sort of thing insofar as it has a lot of built-in stuff that a novel(ist) might want to make use of. Some of the more obvious things in addition to your list:

• 'plain' and 'fancy' anonymous text breaks (ch. 6);
• facilities for styling quotations (ch. 8);
• facilities for changing margins for stretches of text (ch. 8)
• a different assortment of 'page' notes: \sidenote, \sidebars, \sidefootnotes (ch. 12);
• epigraphs (ch. 13);
• various facilities for styling poetry (ch. 14);
• 'editing' markup (ch. 18);
• and, most importantly: the class provides many 'hooks' for you to access the class commands and make minor adjustments to them without needing to reinvent the wheel.

Now, you don't need memoir to do any of this, but using a feature-rich class like this makes it easier to do most of these things unless you already know the equivalent packages and their commands. And since memoir incorporates many of these packages anyway (and were initially written by the same person), the gain is not always there to be had. (The same is true of the KOMA-Script universe, but it was designed with different principles in mind; you really need to pick one or the other for a given project.)

In my experience, memoir can just about do it all, which makes it very convenient. That said, however, you may find that you prefer to use some particular package to the built-in commands memoir provides. (E.g., I sometimes use geometry in this way; and I often load enumitem if I'm using any lists.) You can do this, of course: if you couldn't, memoir would not be nearly so popular. In fact, if you'd rather use one of the packages the class emulates (or need to fix some kind of package option clash), you can use the command:

\DisemulatePackage{<package name>}
\usepackage{<package name>}%
% an example can be seen here:
% http://tex.stackexchange.com/a/134913/8528

But this is likely to be a very rare problem. I myself have never had to worry about it, and I use memoir much more than any other class.

For me the time is most crucial. With Memoir I can do things quicker and somehow easier. Memoir is intuitive to use.