So would you want all classes added, but only simple class definitions? What if your css class was nested? What if it was applied to a specific tag? What would you want to show as the name of the class in the list? 
I would want the classes that I selected to be added.
I don't use templates when I build for clients, so the class names are whatever I choose them to be. I have now got into the practice of naming styles in a 'client friendly' way, if there is to be a CMS or any RTE facility in a client's admin. The actual appearance of the class is also demonstrated in the style select list itself. A somewhat large proportion of the people likely to be editing in HTMLEditor will be sighted, and will see the styles.
I am mindful of this as I build a site. If a class is nested, then I would add the secondary class (.xxxxxx .yyyyy) as a class in its own right as .yyyyy. Whether the class was originally nested or not would be irrelevant.
I have also in the past created a separate stylesheet (copy paste) with just the styles for the select list in the editor. But now I have to enter each style - style by style, property by property all over again.
