Speaking of editing...
Currently Janitors can look at how a node was originally typed-in, and make judgements as to how it was intended to be formatted, even if formatting is missing. They can also look at a node where formatting was used, but used improperly, and quickly ascertain what needs to be done to repair an unreadable node. Imagine if we had to learn all these different flavors of markup. I know you mentioned that the alternate markup form could persist only until the node is committed. But that presents its own problems for Janitors: Nodes that have passed through some sort of preprocessor probably won't retain the original characteristics of how they were typed-in. In other words, we may not be able to see where the author hit enter expecting a new line in his post, if the node has passed through a preprocessor. ...that's just one simple example.
So if the node persists in its typed-in format, we'll have to learn multiple flavors of markup. If it doesn't persist in its typed-in format, we won't be retaining the node's original behind-the-scenes characteristics. Either scenario presents its own problems for Janitors.
Now putting all that aside... If some sort of alternate markup were to be implemented here, rather than making it an explicit "user settings" checkbox, how about making it a tag at the beginning of the post, such as:
<!-- use Markdown --> The rest of the post goes here, in its alternate layout. <!-- use HTML (the default) --> Everything after this point can be typed in HTML format.
The advantage to this is that an individual node could be typed using several markup formats, switching between them using some sort of tag. In practice, people who prefer one format could simply include the necessary tags for that format in their node template, also contained in user settings.
<code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>