Dark Pit is an interesting character choice as he does not need bring much to the table, that is precisely why he is a great potential clone character. Clone characters use a predetermined move-set in a different way, they offer something unique through their similarities. 'Tis their purpose on the roster, good sir.
However, if you are wondering what he does different to warrant the slot, it mostly pertains through his use of different weapons and his association with other characters. For instance, Golden brought up that there is a direct correlation to Pandora, so he could perhaps use Pandora's claws. I thought that idea was interesting; however, most Dark Pit supporters cite his Dark Pit Staff or the different properties on his Silver Bow. The Three Sacred Treasures are never seen with Dark Pit, albeit, nothing stops him from using them. I enjoy his infinite power of flight that is predominantly associated with him (whereas Pit needs Palutena).
Other than the tease, he has evidence on par with Shulk. Tweets, pictures, and more tweets. Tweeted pictures even! His appearance in the trailer is not relevant, it is the unprecedented cliff-hanger compounded with the ambiguous response and appearance in the official art-work that sells him.
He would be his own character for the same reason that you cited.