DeeCee - 4:44 am on Mar 6, 2012 (gmt 0)
The crawlers that go for only the front-page (and sometimes the favicon) are typically info-scanners. Tracking for basic information about the site. Analytics IDs, affiliate IDs, site meta description, site title, a snapshot of the site, and other stuff to add to the information they already loaded from the domain registration information. Info to sell on their own site, about your site. Such as the D*****Tools scanner, which is illegal to mention here. :)
Full content scrapers typically arrive in the middle of the system, hitting on pages loaded out of the Search engine APIs, and they often never even touches the front-page.