Most rotary cutters that see moderate to heavy use will need welding somewhere on the deck soner or later, especially R/C's mounted on a SSL. Welding galvanized steel emits poisonous gasses and is not recommended without specialized personal safety equipment. Given proper equipment storage, rust-through is not a problem for machines built of heavy gauge steel, and rust protection is the primary advantage of the galvanizing proces.
Bottom line, I think a galvanized rotary cutter is a gimmick and a distinct disadvantage that I would actively avoid.