Do you use Outlook or Outlook Express?
Do an eMail message with HTML mode turned on an then go to the source tab. What you will find is all kinds of code that is not what you would intend. For example, it will put in a <div> or <p> code where really you just wanted a <br />.
It is impossible to set up "modern" XHTML since it puts <font> codes everywhere and does not do styles. The only place where I would use a wysiwyg editor is for creating tables, and even then you have to go in and do a lot of editing of column widths and such to get it right.
The other thing the some wysiwyg editors do is insert proprietary codes - that is just plain bad and won't validate according to standards.
The last point that I want to make is that only by looking at the structure of HTML can you appreciate how all these parts work together. HTML should be written based on the semantic structure of the content - something that YOU know, but that a wysiwyg editor simply can't fathom. Here are a couple of links to ponder:
http://www.brainstormsandraves.com/articles/semantics/structure/
http://hotwired.lycos.com/webmonkey/02/42/index4a.html?tw=design
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