Scoring matters, but defense is just as important and Parker is a very bad defender.
Defense isn't as important as scoring when we're talking about individual players, and there's no reason to believe Parker will always be a bad defender, though he may never be a great one.
Uhhh. What?
Let me put it to you the way it was explained on the Lowe Post podcast recently, which I thought was really effective.
Would you rather
(a) be stuck with a really bad offensive player, e.g Omer Asik or Kendrick Perkins, taking the shot every time down the floor,
or instead
(b) have a weak defensive player, e.g. Doug McDermott or Zach Lavine, defending the guy taking the shot every time down the floor?
The team stuck in situation (b) has a much better chance of winning the game.
Asik or Perkins might shoot 20% for the game, and that's optimistic.
The guy trying to score on McDermott or Lavine every time, even if he's on fire, is gonna shoot something like 55-60% probably, and you can always use help defenders or double team, because defense can be a team effort in a way offense can't.
To put it broadly, it's a lot easier to design a team defensive scheme and coach up a group of flawed guys to play good defense than it is to take a group of good individual defenders with poor scoring abilities and turn them into a decent NBA offense.
Consider the Sixers of last season, for example. They were a borderline top 10 defense despite having very little NBA talent on the roster, but the offense was historically terrible.