Why even introduce him if he was just going to die after a few chapters?
Well he was there for the growth of Luffy's and Ace's dream. They all dreamed to be pirate king(Ace changed his dream to help WB), but only one can reach that goal. Part 1 Sabo died and at the end Ace died leaving Luffy to live out there dreams.
-Sabo was a character to build Luffy's morals and goals, to display the loss of family creates a strong will and determination. If you look at the crew members they all basically lost there family and it was a growing moment.
As for Barto I feel he is some what close to Saint Charloss one of the world Nobles. He was the one that Luffy punched in the face for shooting the octopus guy.
-The reason for this is the fact he would hold so much hate for Luffy.
-Also if you noticed Charloss picked his nose a lot, just like Barto does. When we first saw Barto he was picking his nose just like Charloss.
---I know its far fetched, why would a Noble become a pirate? Who knows?