Let's say you have 60 points and gain 20 per win and 20 per loss.
Win streak: 60, 80, 100, Win Win - Promoted 0. It takes 5 wins to go up 40 points. You do not benefit from LP gain in Promos.
Loss streak: 60, 40, 20, 0 - Demoted 75. Takes 5 losses to go down 85 points. You are harmed by LP loss in promos (yes, it's true).
Promos factored into this example, you win an average of 8 LP per game and lose an average of 17 LP per game. For a game that averages out at 50/50 W/L ratio, this seems a little stupid.
tl;dr: Promotionals have been stupid since they've been added in season 3 - still stupid.