This rather long letter on tracking the source of candidates makes a number of interesting points and is certainly worth a read.
When attempting to calculate ROI on campaigns, measuring candidate source makes sense. But as with all analysis, data accuracy is key. If the majority of your source tracking is recorded by recruiters and outbound activity (which in headhunting teams it will be) source tracking provides a useful data point and an unbiased, accurate picture of where your recruiters are investing their time. This will allow teams to spot good and bad trends which in turn will assist in driving diversity. However, a word of caution, when it comes to inbound candidates and applicants, using source data to track where you should be spending your money is significantly more risky.
Having an accurate source for every candidate, let alone a full understanding of how and why the candidate came to you, is fraught with all kinds of variability, especially if you are a large company.