So lets say you have a door way that is large enough for 3 players to stand side by side. Then you have a second line behind them. With player CD the front line cannot get pushed back due to second line and the healers standing farther back keeps the first line healed. It now doesnt matter if there is 20 enemy trying to break through or 200. None will be able to knock back, push through, or target the others inside the building (due to line of sight rules).
If you now apply player CD in PvE then you can have entire guilds blocking boss areas, dungeon entrances and in PvE, towns and cities you have random players in doorways or other entrance areas and you cannot fight them to try and clear the entrances. Player CD is one of the biggest grief play tactics. Player CD is for hardcoe PvP and thats not what the majority of players want.