I think it is safe to say that Jim Smith gets an exceptionally good deal at his MB dealer, but that most of the dealers I've heard of only offer 10% off list for MBCA cards. The one in Annapolis MD (since changed owners) used to give you the "look" when you asked for the discount. That dealership fancies itself as rather trendy, being in Annapolis, and was never competitive with other DC area dealers on price, IMHO.
If you can get competitive prices from a dealer, you're obviously better off due to the contact with the service dept you can maintain. Although if you own older models, some dealers don't have the depth of experience you can find in the independent shops.
I think this is because the older cars are seen less in the dealers and the independents are populated usually with former MB mechanix who were trained on the older models when they worked for MB. Just my opinion, but it is consistent with my experiences.