simple geometry is right; the thicker the blade the less efficient it is at cutting, so what is the correct thickness? The task determines it. .25" blades have uses as do .05" blades.

Most of my everyday carry blades have very thin edges, because they cut paper, cardboard, tomatoes, and things like that, and they cut these things very well. As whats being cut changes, so does the edge. My choppers have thicker edges, and my general users have medium edges.

As jbombelli said, a thicker knife is a good idea because the performance gains in durability far outweight the slight loss of cutting performance. Again the major benifit of a thick chopper is that the added weight. Simple physics tells us you will hit the wood harder, and thus have to hit the wood less times.

The actual edge is a far bigger factor when determining knife performance then the thickness of the blade. You could put a 10 degree edge on a dogfather, and it would cut far better then a thinner knife with a thicker edge.

I feel an ideal bushcraft blade should be around .2" thick. I think every blade that is more then about .23" and is not a big chopper is overkill. a .2" thick blade is thick enough to handle pretty much everything but light enough and thin enough to be easy to carry and "slide" through materials.