Not pertaining to the undercarriage, but there are some posts here in Fitz's thread about rust where a penetration in the van body was cut, and not adequately addressed prior to rust developing. My point here is that if rust can form on the body, it can form underneath the body.
http://www.sportsmobileforum.com/for...-16293-37.html
Whether or not it is a significant issue, probably depends on where you live and where you will use your Sprinter. If either is where they salt roads, or if either is close to the ocean, undercoating may well be worth it to you.
Herb