It's a 'trade' system to ensure the service gets put to good use. You start with 50 SMS points. For every SMS you send, 1 point is deducted, and for every reply you recieve, 2 points are added. See how that ensures good use?
The biggest problem reported with the service so far is that people don't know to reply to the default number sent out.

Edit: this is meant to be a reply to the Google SMS thread. Please merge