It really is this simple:
If you are regularly are wining 2/3 you climb, 1/3 you fall, and 1/2 you stay the same and that is where you belong.
There is no reason to complicate things. Promos create luck in an otherwise balanced system. Let me quickly explain. For every instance you get a troll/dc probability says your opponent will have a troll/dc one game too. This means that over time you will lose 50% of games because of a troll/dc and win 50% because of one so it balances out. Great! Now introduce promos. Promos means that if these random instances happen to you during a promo or two but to an opponent during a regular ranked game or two there is a significantly different effect on your ranked ladder climb. This is why promos are a bad thing. Promos literally can cause some players to have good luck and bad luck, where as without promos all players theoretically have the same amount of luck.
If I confused you, here is an example of what I am trying to say:
Two players both play four games, both starting in their promo series.
Player 1:
Game 1: Win
Game 2: Loss
Game 3: Gets a troll, loses game and promos
Game 4: Other team gets a troll, he moves back up to 98 LP
Player 2:
Game 1: Win
Game 2: Loss
Game 3: Other team gets a troll, wins game and promos
Game 4: Gets a troll, loses game and stays at 0 LP in next division
In a promoless system both players would have been effected the same, going 2-2 and not moving anywhere on the ladder. However, Riot introduced an aspect of luck into their system, where player 1 got unlucky and the troll cost him his promo series causing him to stay in the same division and work back up to promos. Player 2 however got lucky, and got an easy win in his promos thanks to a troll on the other team. Skill-wise the players are the same, but because of Riot's promo system luck came into play.
tl;dr A promoless system is a system that through random probability over time adjusts itself when it comes to trolls/dc/ragers/etc and shows itself to be a fair system that best represents the skill of player.
However, Riot created a system that actually inserts an aspect of luck into a system that is suppose to represent only skill. It baffles me and I wish a Red would explain why now knowing this (if they didn't before) they would continue to use this system.