What are the drawbacks of WYSIWYG editing?
There's nothing wrong with using a previewing editor, as long as you remain aware that it's showing you Just One Possible Rendering, and that, when you're making HTML-based pages, the real product of the authoring process is the structured markup.
Those who want an online simulation of DTP would probably be better off with PDF. At least that _aims_ to preserve the specified appearance, whereas the design aims of HTML lie elsewhere, while CSS is best regarded as a highly desirable suggestion which nevertheless can and must be disregarded when it's getting in the way for some reason (unsuitable display situation, user's special needs...).
WYSIWYG editor additional markup
The extra mark up is a symptom of the problem, not the problem itself. The fundamental insoluble problem of WYSIWYG editors is that the web is NOT a purely visual medium, so approaching design from a purely visual standpoint fails to make the most of the possibilities.
The whole aproach of the WYSIWYG editor is based around the idea that software can be used to automatically mark up a document on an entirely visual basis. But mark up isn't visual. Marking up a document in html is a series of conceptual decisions, exactly the sort of thing that a human being does well and computers can't handle.
The best way to design a site is to get it to make sense first, then to make it as appealing as possible in a wide range of media. Trying to make a pretty picture and then turn it into a web site is entirely the wrong way round.