There's a set of trails around here that has a entire line of 5 jumps, one berm and one roller that is cement. The dirt isn't the kind that would set up well as it has lots of sand/organic material (I"m not a technical dirt expert, i just know the dirt doesn't pack/stay packed)
The locals did a VERY good job of using cement and the jumps ride amazingly.
I'm pretty sure they're doing this to some extent at Catty* in the berms.
* I know they are using cement in some east coast/PA trails.