And I don't know about saving weight or material either. Walls are where most of the material/structure is, if you hollow out an object, you'll double the number of walls needed. Made a paint rack for testors paint bottles a while ago (http://www.thingiverse.com/thing:622033 if anybody cares, but it is that paint bottle rack that testors should have but never made). I made serious attempts to cut back on the material usage by hollowing it out. Turned out a hollow structure was using 10-15% more material than a "solid" one at 10% infill.

That's why with my racing boat parts, I don't add "drill holes" to save weight. The added walls actually adds weight vs leaving it honeycombed.

And while it's very dependent on the exact print, laying infill is vastly quicker than laying another wall.