Well, I had forgotten before but we did get an AskRiot answer directly on it:
Q---When you release a new champ/rework, why don’t you disable the champ in ranked for one week? Why don’t you need a certain number of normal games before you play a champ in ranked?
A---Releasing a new champion or a major champion update is a big deal for us and for players, especially at the slower release cadence we use these days. After the teasing and the hype and the spotlights, we want players to be able to try the champion out right away. For players whose primary way to engage in League is through ranked (and that’s a lot of players) a week can feel like a long time, especially since the champion will have been on the PBE before that.
However, we sympathize with the basic problem you’re hinting at, of someone going into ranked with a champion they have never played before, and having your team possibly pay the price. While we think the new Practice Mode will help some with this problem, we also have discussed requiring you to play with the champion – even if it’s as little as one game – before you take him or her into ranked. No concrete plans at the moment, but it’s something we’ll keep on our radar.
GHOSTCRAWLER
Design Director, League of Legends