Don't forget that many parts are covered to 100k already on the diesels. Also, no matter where you buy your warranty, if you don't stay "stock" the engine/drivetrain warranty will probably get voided.
The extended warranty is a cost-averaging among owners, with built in profit for the organization that issues it to you. Another name: Insurance. If you never buy the warranty, over your lifetime you should come out ahead by cutting out the "middleman", even if once or twice you get stuck with an expensive repair.
At least once in my life, when a salesman told me I "needed" the extended warranty, I told him that showed me the truck must be unreliable, I didn't want an unreliable truck, and I walked out.
On the other hand, if you don't like playing the odds, then the peace of mind that comes with an extended warranty has value for you. Just be sure to read all the fine print and do everything needed to keep the warranty in force, including required maintenance and especially avoiding the power-enhancing mods that are so popular on this board. You can't step up the power and then expect DC to still warranty the drivetrain; they turned down the power of the Cummins for exactly that reason - to have fewer trucks breaking on their nickle.