I don't see what makes you think they'll become part of the gang permanently. Look at previous LBT movies, and there have been other occasions where a new character or two joined the gang on an adventure, and even became friends. But they never became a permanent part of the gang.
Ruby and Chomper most likely will not become permanent members. Remember, Ruby is only there to learn how to stop Redclaw, then she'll most likely move back with her family. And I'm sure Chomper's parents would want him back soon as well when Redclaw is stopped. Plus, due to his species, Chomper can't stay in the valley for too long.
I don't agree that Ruby serves no purpose. If this were the case, then she would be just a side character in the show, only occasionally appearing. But because she is one of the main characters, she definitely has a purpose in the series, along with Chomper.