No, I don't think that is a good idea. Either your user has no idea what they are doing in a structured editing environment, which means they should keep their hands off the content. Or the user can be shown that inserting a <b> element is just the same as inserting a <p> or any other element in a structured editing environment.
If your users have no idea about structured authoring, they should get educated before allowing them to do anything to your DITA materials. Unstructured thinking in a structured environment WILL lead to a total mess. There is no way your users can get the benefits of a structured editing environment without learning the basic rules. Inserting a <b> element to make something appear bold is as basic as it gets.