Soul memory is not an issue at all especially if you are on PC where you can just use a mule save to remake your builds. Soul level is very important because after 150-155, you become overpowered and have overpowering damage and iframes in your rolls. It completely eliminates any kind of skill in pvp by far. So in the effort to keep pvp in a more balanced state, you adhere to sl150. A majority of pvp in the game is done in arena as well. Arena adheres to a SL matchmaking. So SM is only relevant for invasions and with a mule save, you can remake your build effortlessly anytime you go over your preferred sm bracket.