There is a further consideration which the article perfectly illustrates: if you have water in your image, you have to make very sure that it's reflecting the sky to a degree that is believable. The images in the article simply show the sky as reflected when the image was shot but not the skies that were dropped in.
Which brings me to another point. Whilst the 'right' sky can undoubtedly make or break a good landscape photo, what is more important is the subtle lighting change that each type of sky brings with it. An underlit cloudy sky (i.e. cloud covers most of the sky but a small strip of sky is visible where the sun is setting or rising) creates a beautiful warm glow light, for example, which is non-existent in clearer skies and very difficult to fake unless you are a skilled Photoshop artist.
Personally, I don't much go for this type of 'bolt-on' moodiness. The most important thing is to capture what you felt at the time not to attempt to tart-up a so-so image after the fact. That said, there's a case to be made for this sort of manipulation in the advertising world where expensive shoots can't be re-scheduled simply because the weather isn't co-operating but, for the rest of us, most of the challenge of landscape photography is finding (or waiting for) the perfect conditions.