His usefulness in battle depends on how much you have "levelled" your characters, as new party members start with a set route on the grid when you get them. Normally Auron would have the highest Str when he joins. As for later in the game, his Overdrives are a lot less useful than Wakka and Tidus' and Aoens Overdrives. And I guess alot of people like him for being cool and tough. With people I mean emo kids.