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For what it's worth, I wouldn't still be playing LoL if I couldn't play in matches with the group of 4 friends I know that are all level 30 already.
And their reason for queuing with me is to play with me, as well as directly observe me in action in order to have better advise to give me to make me a better player.
To me, it's just an added bonus that I'm playing against level 18-25 players on average instead of player's at my level. It's tougher competition and makes me a better player faster. I don't think my level 30 friends necessarily take this into consideration, but I like it.