In what way did Chakotay and Mayweather not have a purpose? They weren't just totally bland character like Kim or Tuvok, they were quite flavoured (a rebel and a cargo dude born in space!) but the actors couldn't make the roles come alive. And when somebody doesn't give a shit like Beltran and whines about how ridiculous the only show he will ever be remembered for is I have the audacity to point out that such an attitude is the main reason for the failure of this character.
These actors were no George Takeis, never getting a chance to shine until they actually get something to do in the end (TUC). They had their chances (the two Mayweather shows Fortunate Son and Horizon were fairly well written) and blew them.