You'll miss out on the complete picture. Just as if you confined Google Analytics to a few pages, limiting our code will result in the collection of less data.
You won't know if a visitor returned to your website without clicking on new pages.
You'll only be aware of part of the customer journey. This, in turn, may skew your understanding of what a customer wanted when he or she visited your website.
Additionally, you should consider the fact that if you put the code on the entire site, you can still segment the traffic. For example, you can restrict the results to new or used auto buyers, service buyers, and even potential employees (i.e., you can use it for recruiting as well).
Curious about web ID? Click here to read more about how long it takes to work.