Because module method is already a singleton method, and you don't need to do  ::Mailboxer::Cleaner.instance.strip_tags(text), but rather  ::Mailboxer::Cleaner.strip_tags(text), and you also don't need the additional dependency on rubinius. As a rule of thumb, singletons are needed when there's something that's needed to be done in the initializer. For all other cases module methods are a better fit.