I have found two well respected authors that concur on this:
Robert M. Fenner, The Conscientous Marine Aquarist, page 96:
Commercial fish-only systems are frequently kept at artificially low specific gravities (1.017-1.020) for three principal benefits:
1) cost of salt mix is less.
2) parasite and microbial levels are reduced (these life forms can't make the osmotic stretch as well as macrolife).
3) gas solubility or oxygen content is enhanced.
Nick Dakin, Complete Encyclopedia of the Saltwater Aquarium, page 89:
The control of salinity can be utilized as an aid to disease prevention. Many disease pathogens, particularly external crustaceans and protozoans, can not survive in low salinities and it has become common practice to maintain marine aquariums at a specific gravity of 1.022, and in some cases as low as 1.018. Most fishes can tolerate these levels and actually seem to benefit from them because of less demanding osmotic pressure. However, many invertebrates will not do well below 1.022.
These are both very recent publications, too. If anyone has any sources of reference that would contradict this, please post it.