All this talk of maining one or two champions doesn't work in ranked. There will be plenty of times you are last pick and you will be stuck support/jungle/adc or anything you don't want to play. "Calling" your favorite role won't work because people don't care to listen.
So unless you dodge every time you dont get the role you want, you are gunna have to at least be proficient at all of them.
Also, alot of new people here starting ranked, having about 900 matches done in season 2 from ranked alone, I will say it is probably the singlemost frustrating experience I have ever had in a game trying to get to 1600+ ELO
even if you only get the champion you want say 60% of the time when you're doing your first X amount of games it will help out a lot
I noticed it before this but there was a video recently made showing the ELO people have made playing one champion (using InvertedComposer and HotshotGG as examples) and I have to say, they have gained tons of ELO mainly playing Singed and Nidalee (Hotshot playing Nid and InvertedComposer playing Singed)
and if you look at the rest of their W/L it is actually pretty meh, but by gaining so much ELO on their 'mains' they were pretty much able to forget about the bad games they had on other champions
Yes you should get good practice on all roles
but that isn't necessarily how you gain ELO, you gain ELO by trying to do your best in the games you do play and trying to carry
and to do that you really should be playing the champions you are best at