By doing a flush, the dealer doesn't have to tie up a perfectly good bay, doesn't have to worry about a comeback from lousy pan sealing, and doesn't have to deal with anyone moaning about bathing in ATF. They also usually get to soak you for a lot more money—most of them will charge for an overhaul fill, which is 2-3 times the fluid the Jeep-recommended drain and refill uses. Some even charge for any additional fluid used during the flush.
A flush isn't going to magically replace tired O-rings or filter media, and though the additive might break some crud loose from the filter, ask yourself where it's going to go: The flush machine runs the fluid in the same direction the transmission does.
How'd you end up due for a transmission service? Even severe schedule's only every 60K even with the CRD as far as I can tell. I hope you're driving the heck out of your CRD and didn't get the "Oh yeah, 30K, time for a service in these harsh CITY X conditions!" ****-and-bull story. If you chose to do it on the 30s that's fine, but there are a lot of shops that would sell you a tranny service every visit if they thought you'd bite.
2006 WK Laredo 4x4 4.7L
1993 ZJ Laredo 4x4 4.0L — Dead, sold, and rolling again