How I see it:
If you are lets say Silver...you should play against silver to prove you not only deserve that rank, but to also Advance from it.
But because of MMR, you end up going against Diamonds and Plats sometimes (rather because they smurfed it, or that's where you do belong.
The problem with this is the fact going an advancement is always a coin flip because you are going against Skill lvl (decided by MMR), and not based on Rank. Its like saying, "Want to be Gold? Take on on these random plats and challengers for a 50/50 chance to prove you deserve to get into Gold, but you lose more LP then you do Gaining, enjoy."
It is like saying because a 10 year old is good in little league that they can go against a high school players because of their "skill" instead of playing against those around the same age range and rank of Little League.
P.S. For those who don't get it: The point of rank is to fight those of the same rank to advance, not give someone of your skill level who may be in a different/much higher bracket.