Honestly, the reward for your efforts is the mastery score, the reaction from your friends when they view your profile ("Damn! That's a lot of level fives!"), and the mastery banner and added effects. It's a system that only means something if yoy want it to, and I think that that's cool. (I personally love the banners and showing them off, and that's why I've been trying to get my entire pool of champions to level five. (I also am a completist, like you, and I want to be able to say that I have level five on all of my champions.) Once all twenty-one of my champions are level five, I can feel free to play around with them and flaunt my mastery banners.) I understand that you want something more out of the system, but I honestly think that the fun-ness of the system would leave if there were setious rewards at stake.
Here's an example. Say Riot releases "Masterful Lee Sin" for people with three hundred mastery score. Suddenly, I can't feel content with my small champion pool, since even with getting all of my champions to level seven I wouldn't be close to getting the skin. I'd have to start buying champions to play or using free-week champions, and I don't like doing that. I dislike the experience of doing terribly for my first ten or so matches on a champion as I get used to it, and I also don't find much of the champion pool extremely interesting to me. However, I'd <really> like to get Lee Sin and his skin for free, and so I'd grudgingly start playing more champions. Boom. Champion mastery isn't fun for me anymore. Also, right now I'm spending all of my IP on Runes and I can't spare any for champions.
This is why I'd like to keep the rewards the way they are.
(However, stuff is going to change once Riot releases Hextech crafting. Apparently there are skin rewards and the system has to do with champion mastery, so you might actually sorta get what you want. It might be something like a low chance for skin rewards for players playing on champions level five and up, with slightly higher chances for players with champions levels six and seven, or something like that. Whatever it is, it'll probably benefit you to have all of those level fives, so you'll probably get some nice reward for your dedication to the mastety system.)