Just like the actor who accepted a role in movie with an ill-conceived plot, so too is the mystery writer sometimes hurried into finishing a novel before its characters or plot are fully developed.
Mystery readers, however, are generally much less forgiving than moviegoers. We expect only the best from our favorite writers and are sorely disappointed when they offer, perhaps a "B-" book, instead of the "A" to which we've become accustomed.
I say that perhaps we should be more lenient with that one book out of a dozen that's not quite there or the last in a series from an aging author who's not the writer she was 30 years before. If we've enjoyed their writing over several years and books, we owe them that one faux pas.