This has been brought up in the past. I think it wasn't added to keep it bit more rewarding and satisfying when you do manage to dismember something. Without knowing when something is going to be dismembered, it also makes the combat a bit more of a challenge. The idea didn't seem to get very far.
As for single player replay editing, I think one of the goals is rewinding and fast-forwarding. Which would be much more convenient for perfecting a move.