How Thin is Too Thin?

Hey, I'm a n00b here. Got the bug to build a TD and am hoping to build one someday soon. So my first question; how thin is too thin? Many builders say their first build was overbuilt and way too heavy. I want to avoid that mistake.
It seems there are two popular methods of construction. The heavy builds utilize a wooden frame and the lighter one sandwiches a sheet of .75" ply, removing sections to reduce weight.
My idea is to recreate a woodie look with an exposed frame of structural hardwood (1"x2" birch?) layered over a sheets of .25" marine grade plywood, Celotex and beadboard paneling.
As I see it, the structure only needs to hold together going down the road. Assuming the birch exoskeleton is constructed properly, (with some interior cabinets added for good measure) it seems like I should have no problems. However, all of you who know way more that I do don't build them this way. Why not?
It seems there are two popular methods of construction. The heavy builds utilize a wooden frame and the lighter one sandwiches a sheet of .75" ply, removing sections to reduce weight.
My idea is to recreate a woodie look with an exposed frame of structural hardwood (1"x2" birch?) layered over a sheets of .25" marine grade plywood, Celotex and beadboard paneling.
As I see it, the structure only needs to hold together going down the road. Assuming the birch exoskeleton is constructed properly, (with some interior cabinets added for good measure) it seems like I should have no problems. However, all of you who know way more that I do don't build them this way. Why not?