Its a 2 way street. All the new missions and strikes and raid require it. But i reached lvl. 31 without buying it. And most people have it so when youre on a fireteam and everyone has the DLC but you, you feel left out and have to leave the fireteam.
Depends how much you play the game and whether or not you have friends who are active enough in the game to be a raid team? Honestly, if you don't have a full raid team and/or don't play destiny very often, I'd wait until the price goes way down from $20. I'm not sure the amount of content in dark below could ever justify a $20 price tag, no matter how hardcore of a fan you are.
Hope this helps your decision making, friend.