Streetball is a game of short offensive bursts. The greater leeway for dribbling violations, the player-refereed foul calls, and a much shorter duration of game play, makes offense much more favorable. You can go up 5-0, miss a few shots, and ultimately lose 5-11. If you can’t make the other team miss, you will be playing defense all the way until game point.
In a case of basketball irony, the weaker you are on defense, the more your opponents will try and attack you on offense —- forcing you to play even more defense! You won’t be happy having to fight through screens, or having someone back you down at the post for every other possession. Your team mates certainly won’t be happy sprinting over to cover your ass when you let your assignment get pass you. Being a reputable defender — or even a decent one— would make your opponent reconsider going at you, passing up the ball to his team mates instead.
Everyone will have bad days on offense, but defense will always stay. Being a good defender means that you’ll always be able to contribute, a constant base value if you’d like to call that, even when your shots are not falling. That makes you a far more valuable option when choosing team members, because other people know that you can constantly bring good ‘D’ to any game.