The reasoning is when calculating for deflection (bending due to load), unbraced length is taken to the forth power. 16^4 =65536 24^4 =331776 The difference in deflection can be roughly estimated relative to eachother. 331776/65536= 5.0625 The deflection on a plywood sheathing between studs is 5 times higher for 24" o/c vs 16" o/c. I just don't see how that loss in rigidity is worth the maybe $800 in lumber savings over the lifetime of the shed.