If foam alone is good enough for walls..

I have been on the forum for about a week now - meticuously planning my project. I have only heard about the foamie method a little bit and just came to this section to check it out farther. I'm blown away by the fact that so many of you have built these things out of mostly foam board. I plan to use foam insulated walls with playwood on either side of that insulation. I had been weighing the ups and downs of using 1/8" ply, 3/8" ply and 1/2" ply and even thicker for the walls and roof. Now that I'm seeing that you can actually use ONLY foam, I'm thinking a bit differently! I don't see why I can't get away with 1/4" ply on outside walls and 1/8" ply for all the rest - ceiling and interior. My trailer is larger than a teardrop however.. It is going to be 10 feet long and 6 feet high. More of a micro travel trailer - one that you can stand upright in.