According to http://area51.stackexchange.com/faq, there's a reputation bonus for a referred user following through with a commitment, but there's no bonus for following through with the commitment yourself. It would make sense to receive a bonus for following through yourself, to encourage actually following through. Similarly, it may make sense to have a penalty for not following through with the commitment.
This has been implemented. As the FAQ now states, you get 50 reputation when you fulfill your own commitment, and 25 reputation when a user you referred fulfills theirs.
The problem with a penalty: what happens if the site turns out not to be what you expect? After all, what's happened is that some sample questions have been argued on, and then nothing happens for some time while people you don't get to know join. Once the private beta starts, it might be functionally a different site.