Agreed. It seems from what I've looked at is that if everyone is limited to just playing with their rank, the highest ranks would have very long queue times. If that's an issue, then allow the highest ranks to que with a few ranks below them, but not with the very lowest ranks.
Or just have it so ranks can play with a couple ranks below/above, so the que times isn't an issue, and people can still face people a bit better than them. Sometimes it's good to face people a tiny bit better to learn (though facing the highest ranks and just getting completely decimated doesn't help learning).
I feel it would help keep the newest players playing league if they didn't end up with the highest ranked people in their games, it causes league to not seem very beginner friendly. (though it does take skill to learn and master). Who wants to keep playing a game where you just keep losing and having no chance to win?
If we knew more about mmr, or if it was improved, perhaps it could work but right now as mmr is, it really doesn't seem like a good way to put people into games.