Due to how converters are implemented, when a meta-type (like richtext) is used, often double conversion occurrs. The first conversion is from the underlying type (like string) to the meta type, the second is from the metatype back to string to display it. When the content is modified by a form, it again happens twice.
If conversion is rather simple, this poses no problem, but the whole idea behind underlying meta types is to offer better conversion and validations, so most probably converters will not be that simple.
This can be desiderable in some context, for example if I'm migrating an already existing database that contains rich text fields, but previous contents were unfiltered, filtering them upon load can ensure proper formatting of the page, or while changing validation settings dureing developement.
But it is overkill for most situations, and wastes a lot of cpu time parsing and reparsing HTML.