Down-voting is explicitly discouraged by the system; there are warnings and XP penalties if you do much of it. So it's no surprise that people get the message that -- is bad, and start preferring ++ instead.

While I share your pain at the low quality of some contributions, I fear it's inevitable, regardless of the voting system. From what I've seen on other forums, people would probably jump in with poorly-thought-out, borderline-irrelevant replies even if they weren't going to get any XP out of it. Some people just go for "high scores" instinctively, and if they can't get XP, they'll go for high post counts or "first posts" instead -- and that's arguably even worse!