I think this is a good rule, it encourages people to write longer and more thoughtful posts. This is a message board, not a chat room. It's designed for people to leave messages for others, not 3 character replies.
There is this rule and the 30 seconds between post rules that help to deter those outrageously shallow threads about "live match discussions" with one-liners back and forth. In a chat room setting, it would be easy to follow such a conversation, but it's a pain in the butt to read on here.
After watching the Hewitt-Federer final, I came on here and saw one of those "live discussions," which was like 200 messages long. I started to read it from the beginning but simply couldn't read it all, it was all stream of consciousness drivel (well most of it).