Aside from being a big friendly fella, his hatred for the Al Bhed and his devotion to Yevon means he's kind of representative of most Yevonites. I think having that sort of character in the party was important for the plot and aided in the players understanding of Spira, particularly as Yevon's true colours start to show.
In battle, he's awesome. Strength just about on par with Auron, best Accuracy, status effects and most damaging Overdrive.

One thing that's always bothered me about Wakka is the fact that he uses a Blitzball in battle.
I mean, he's on a team that hasn't won for 23 years right? I know he wasn't part of the team the whole time and probably only 'turned pro' a few years before Tidus washed ashore, but nonetheless... Statistically, he was one of the worst Blitzers in Spira when we first meet him. So why the Blitzball? If anything, Tidus and Wakka should swap weapons. Wakka had plenty of time to learn how to use Brotherhood since Chappu left it behind. Presumably there's not much else to do in Besaid. And Tidus is the superstar right? Why just have Wakka throwing the ball at fiends when Tidus could've been Jecht Shotting it every turn?