There's no reason to do it other than to prove it can be done. I mean, it can work if you got the chops, people win games with a bot Jayce, but on a fundamental technical level he's just not as good a bot laner as any other actual pure ranged carry.
And I say this as a guy who really really likes Jayce.
If you do want to do it though, for whatever reason, then Janna is a decent choice because Jayce loves his AD. You really want a partner who has a good sense of when you're going to poke though, because you don't want to get shielded half a second after you shoot a cannon Q. --- "ˇViva La Revolución!" -Salvador Limones