I don't know about the whole 'leads capturing Legendaries' thing. I mean, yeah, there have been times where Ash has actually won against a Legendary Pokemon, and I guess we could take that as meaning that it's possible that he might get to that point.
But here's the thing. Legendary Pokemon are very difficult to use in the general sense. Normally when a Legendary Pokemon comes into the mix, it's because there's a **** good reason for it being there. It usually means that there's some kind of major event about to happen.
Now let's say that Ash gets a Legendary Pokemon. Say he gets Ho-Oh. Alright. Now he's got this ridiculously powerful Pokemon that can destroy just about ANY Trainer in the Anime that doesn't have Legendary Pokemon. So now everyone will have to start getting Legendaries in order to balance the equation. With FanFics, it's fine, because each story is its own thing. But in the mainstream Anime, where Ash's journey has been going on for over a decade, that could really change things permanently. So it would have to be handled with the utmost care.