The main reasons for "requiring" (note that we do not have a strict prohibition on drive by posting here, we take it on a situation by situation basis) someone to add their own commentary on what they've posted:
1. Obviously, if they took the time to cut and paste, they have some feelings in the matter. Why be coy about it?
2. Mutual courtesy. If you're too lazy to add some of your own thoughts to something you've posted, well then maybe I'm too lazy to bother to read it.
3. Snipers. At some other boards, there were a few people who used this tactic only to start arguments. They'd post blind, wait for someone to comment, and then come down on them/their position like a ton of bricks. Not unlike the Monty Python Argument Sketch, only with none of the humor and 10 times the stupidity.