You and I see the Posey collision very differently. He was not blocking the plate, the runner ran inside the base path to hit him when he could have easily slid around him.
But that one hit, while awful, is not the sole reason I don't think they should be allowed. To me a close slide is no less exciting than a collision, and when you factor in the types of injuries that might result from sliding versus collisions, it's just a no-brainer for me.