The difference in composition between 316 and 316L stainless steel is that 316L has lower carbon content (< 0.03%). As you pointed out, the lower carbon content is only important to corrosion resistance if welding is involved. Because of the higher carbon levels, welding 316 (or 304) can lead to sensitization and precipitation of chromium carbide at the grain boundaries making it more susceptible to intergranular corrosion. Beyond better welded corrosion resistance, I am not aware of any reason that 316L should be superior to 316 (or 304L to 304). It would seem that the low carbon versions of stainless steel would not be necessary for valve flanges unless welding is somehow involved in the valve manufacturing process.