Although i hate getting promoted then losing 4 games ( do to afk's, trolls, me playing bad ) in a row just to get demoted back, it helps you as a player. If you can climb up again that means you deserve to be in that division.
I'm in platinum 1, I've loss 2 of my diamond promos and have been being demoted and re-promoted back from platinum 1 and 2 ( i Cry )
The system is fine, (frustrating as it is) It does its best to weed out who belongs and who doesn't. If you had a bad run and got demoted, get back up and keep playing. For me as a frequent top laner, the decision to hit a creep or not to hi creep determines whether I win or lose lane. Don't play to win, play to improve and you'll climb.
Even if the game is a 4v5, You still could of won the game. Did you die in lane? Did you forget to ward? Did you roam enough? Did you focus objectives? Did you flame? These are things you should think about every time you lose.
It might sound cheesy but who ever wins a game is not the team that won. It's the team that made the fewer mistakes then the other.