Why is a work being unpredictable a bad thing? Isn't the fact that it's unpredictable an indication of it having not just one plot twist, but many?
Keep in mind that I'm talking about completed works that have gone through the revision process. I am not talking about first drafts that have errors in them that render them nonsensical (as most people seem to be assuming).
Planning does not have to be done before you begin a project. It can be done after the first draft is written, when you have a clear idea of how the story on a whole is and can see how the story on a whole can be improved and tightened, when you can see how the various subplots and story elements work together to enhance or distract from the main plot. You can then plan better ways to foreshadow future twists, or better ways to tie in the subplots, or even plan to take a particularly strong subplot and make it into the main plot.
Where in the writing process you figure these things out is the only difference between planners and spontaneous writers. Planners figure out the major parts of the story before writing it, and then refine that plan during revision. Spontaneous writers figure things out during the revision process, and refine the story that results.
If we use the analogy of carving a gemstone, planners set out to carve the stone in a specific shape they decided beforehand; spontaneous writers take a look at the stone for what it is and then, based on the stone's characteristics, carve it into a shape that enhances the already-present features of the stone.
Both methods result in a carved and polished gemstone.
