User blog comment:Odisseas25/Why does the AP go to mid?/@comment-4834593-20130210132349

The AP goes solo because he scales with levels, and this solo lane is normally mid, because you want many people around Dragon, so either mid or bot is the duo lane, and mid lane is a shorter lane, which makes a duo lane more crowded, and the support also can't poke as easily.

And for solo queue it is also really useful to have set lanes for everyone, because otherwise you don't get balanced teams, but in premade teams, swapping lanes can be really effecient. I've also seen a pro game once where the support + AD carry went mid, the AP went top and the bruiser went bot. Or something like that. I don't remember exactly.