Here's what Salesforce had to say about the rationale for acquisition:
The process of trying to acquire, complete and cleanse business contact data is far too taxing and inefficient to meet the modern needs of companies in today’s competitive marketplace. Historically, companies have spent vast amounts of time and resources to build a database of complete contact information for their sales organization only to see its quality quickly deteriorate. With the integrated Jigsaw and salesforce.com solution, companies will be able to better acquire and cleanse all business contact data. While Jigsaw will remain open to all CRM systems, current salesforce.com customers will find an integrated experience that includes an automated process for data cleansing. In addition, since Jigsaw products are already integrated with salesforce.com technologies, when Salesforce Chatter becomes generally available later this year, users will be able to receive real-time feeds when data changes or requires cleansing.