I thought the first post should be dedicated to the appearance of a recent book evaluating David's descriptive definition of Evangelicalism: The Emergence of Evangelicalism: Exploring Historical Continuities (Nottingham: IVP, 2008) edited by Michael Haykin and Kenneth Stewart. (You can find information on it here, here and here). Unfortunately--for all you US based folks--it is only available in the UK at the moment.
'For example, if no context is made explicit, an argument could be made that St Francis of Assisi was an evangelical. St. Francis, after all, had a clear, dramatic conversion experience; he was so committed to activism that he pioneered friars out itinerating amongst the people, preaching the gospel, and ministering to physical needs rather than being cloistered monks; his biblicism was so thorough that his Rule was made up mostly of straight quotations from Scripture; his crucicentrism was so profound that it reached its culmination in the stigmata.' (T. Larsen in The Cambridge Companion to Evangelical Theology (Cambridge University Press, 2007), 2.)
Yet, no other definition of evangelicalism comes close to rivaling the scholarly acceptance of David's quadrilateral. I think many people get hung up on the descriptive/prescriptive qualifications when examining the definition as it is configured in the quadrilateral. As historians, we are not (or should not be) attempting to place a prescriptive framework on the past. We hypothesize but then look for data which will make or break our hypothesis. My own thought on the quadrilateral is that although it possesses a few minor cracks, it is a solid descriptive definition of Evangelicalism.