Yes I realize how important the role of Summoner is, but that's the only thing that makes Yuna important. She would have been basically useless as anything else (not in battles though, I <3 white magery ). As I've been saying, she did not take up the leadership position, despite being the centre of the group.
Yes, Auron does first come into the group at Luca (ignoring the Zanarkand scene). However, that is still very early in the story, and very little had been decided by that point at all. I honestly can't give any examples of big decisions Auron makes... I wish I could, but it's been a while since I've been really stroyline-oriented in this game. I do know though, that his guidance... his leadership if you will, helps the party ALOT through the middle section of the story, and as I've said, he is the major decision maker for quite a long time.
It seems to me, all you're doing is downplaying Auron's role by switching words. What he says could be taken as suggestions, but since the whole party knows to trust him, and therefore follows his advice for the most part, I prefer to call them decisions, and Auron a leader. Tidus was the 'helper', and, though Yuna did make some big decisions, she usually sought Auron's consent beforehand anyways... recognizing who the leader was...
EDIT: WHOOOOAAA, I just looked at the title of the thread again. Hah, I'm drifting pretty off-topic here, I'll try to focus on Tidus more than Auron from now on then. Still, what I've been saying is why Tidus isn't a leader. It's not like that's a big problem though... really, it's a nice twist away from the protagonist=leader thing I see all too often in RPGs (though, not being a real hardcore gamer, I can't say that with any conviction..). I don't like Tidus for other reasons though, which I'm sure I've said earlier.