I have replaced a couple of the luan slab doors in our house with oak faced slabs. This is pretty easy to get right if I just use the old slab door as a template for trimming the new door, mortising the hinges and drilling for the lockset.
If I replaced the jamb too, I would have to remove and replace the casing, level and plumb the new jamb, etc. and then remove the hinges from the new door for finishing. I might still have to trim the bottom of the new door to match the carpet.
In both cases, I am not allowing the week or so for me to finish the new door with a couple of coats of stain and poly. But doing it this way, I can keep using the old door until the new one is ready. If I would pull the jamb, I probably would not have a door in place at all until that last coat of poly was dry. I recognize that the cost of the pre-hung is only a little higher but even that adds to the negatives for me.