Heterocysts are expensive! When heterocysts are present, most of the energy used by the filament goes towards (1) making the specialized heterocyst envelope, (2) providing reducing electrons to nitrogen gas, and (3) maintaining the oxygen-free environment required by nitrogen fixation. Furthermore, since heterocysts don't divide, they consititute a genetic drain. Optimally, heterocysts should be spaced so that the minimum number has the maximum effect on feeding the filament fixed nitrogen.