If you're trying to practice, wouldn't you want to play against higher rated players? I don't understand the problem there.
Being Plat, I love the opportunity to play with higher ranked players but I also want to be able to practice/test things in an environment similar to the one I compete in. My issue with premades has less to do with playing vs the occasional diamond and more to do with having to play with a handicap on my or the enemy team.
Even if it's just a bronze support on a team with four platinum ranked players, that one player is a point of weakness; not warding correctly, not calling movements, misusing abilities and miss-positioning.
Most of the time I end up playing vs someone who's still trying to learn the game. When that happens he has a bad time because he's playing a mismatch that shouldn't of happened in the first place and I have a bad time because I'm wasting time pushing around new/low-tier players when I wanted to practice/test.
In the rare games where I lane vs an even match I enjoy the opportunity to really hash it out and practice but then the mid/late game happens and it comes down to "who's team has the dumber new player". Practicing LoL means more than just practicing lane mechanics. Testing the late game viability of certain champs is a moot objective if I go even in lane but my low tier jungler has been feeding the enemy ADC double buffs all game.