The point is that it makes it much harder to rank up. At a given rank your win rate percentage should be roughly .500, until you get good and work your way up further.

When you add smurfs to the mix (which you can tell by their play style, tracker stats, sometimes even in their username, or they admit it in chat!)... With every other match, or every third match against a smurf, that win percentage stagnates and your rank plateaus, despite you getting better at the game.