I have wondered this since i started playing. I don't understand why they have a premade 5's que but it still only ranks you as an individual.. Just an ingame clan/team system. its easy as hell to do in terms of coding, they already pretty much have the basics down for it. (with chat groups, blahblah) Only would have to implement it into the premade ladder.
Will Riot do this? No. Would riot do this if they said they would? No.
Just stick to online ladders(i dont know any good ones for anygame ever since TWL turned to ****, CAL never grabbed a game other than CS that was worth a ****.) But those work just as good.
EDIT: Not to mention you get individual rating but so focused on team play. Its literally impossible to carry a team -1200 ELO