I feel the extent of the Senju and Uchiha's relivance to the series is only to be recognized as the strongest clans in the history books.
Although they were revered as strong I don't believe Kishi ever meant to display the powers of the average Senju or Uchiha. They are the stories of strong civilizations that nobody will ever see but imagine in their minds and wonder. It's that factor about the two clans I love.
However with teh Uchiha's, Kishi's own interest in that clan in perticular has given them power and displayed numerous characters (Madara, Itachi, Shisui, Sasuke) above the tier and they act as the faces of the entire clan.
Hashirama acts as the face of his clan. These faces are merely to give the idea of the extremes of the clan, but not the general idea of the average strength of each member. THe faces are to show the highest potential of each clan, which is above most if not all. We get a sense of how what each clan is about from their reputation and stories. With the exception of Itachi and Shisui the clan's been about hatred and vengence.
However those two clans are legacies mainly, the legacy of the Senju is the will of fire, while that of the Uchiha is hatred.
The reason why no Uchiha or Senju was brought back was to maintain that sort of legacy aspect, we get an idea of what the Senju and Uchiha are about but one or two examples of their power in order to keep the ancient revered cultures of the two clan's intact meaning everything is up to the viewer to determine how great the shinobi that came from those clans were.