Narrow boards glued together into a wider board are not "laminated." In wood, lamination refers to layers glued together in thickness, as in plywood. The frame helps prevent the panel from warping because the rails apply a cross force to the glued-up boards in the panel. This is the major reason for the traditional design, not just to arbitrarily make more work for the cabinet maker. Battens on the back of a slab door provide a similar cross force. Aprons on a table, likewise. In virtually any traditional design with wood, that cross force is applied in some way. Honestly, I think it's a bit silly to "hate" the battens on the inside of a door that prevent it from warping. But if there is no batten, I would choose veneer over plywood for the slab, making sure the same species and thickness of plies on both front and back to minimize warping. MDF is very heavy and often has hinge failures. And be much more careful than normal to make sure the doors remain dry and the finish intact. The idea that you can find cabinet makers who don't use traditional design, or provide warranties, is no assurance the new design will hold up. In the old days, longevity was more valued and expected. (Some) cabinet makers today know that the average kitchen is replaced in X number of years because people move or want a new look. I find that in home construction in my area, it's possible (easy even) to find workers in every trade who will do work in a way that is highly likely to fail prematurely.

I've used Masonite Capri solid core 80" and 96" doors in builds. Never pre-hung. Three heavy-duty decorative hinges were always sufficient. But I would check that the hinges are true loose-pin, so the doors can be easily removed for trimming as the house settles or to ease moving day concerns. Some loose-pin hinges are lubricated for life with materials that make removing the pin impossible. And you don't want that stuff on your bare skin!

Thanks to everyone who replied! I really appreciate your feedback. Couple of additional questions, if you have time: 1. Do you feel it is a bad idea to paint Douglas fir, rather than stain it? My husband thinks I am crazy for wanting to paint stained doors. FYI: The 1985 house that we moved into has stained molding, doors, and trim throughout. I am updating the house with taller molding and painting the molding, doors, and trim white. I'm also purchasing Emtek Delaware levers with sideplate: https://emtek.com/sideplate-locks/delaware-9-nonkey. 2. I'd like to convert all of our bi-fold closet doors to French doors. Again, the contractor tells me that it would be cheaper to purchase a pre-hung, solid-core door made of wood veneer than to convert the old doors. However, our current doors are solid Douglas Fir, and I don't want to replace with cheap doors. I've included photos. Thoughts??

You're not ordering door slabs machined for you existing frames over the internet. They will never fit correctly. They need to be measured and fit onsite as the door frames will never be perfectly square. The news doors will need to be trimmed more than likely a bit of of square. Best bet is new prehung doors from a lumberyard or reputable millwork shop, definitely not a box store.