The buff effects increase substantially when the accessory is maxed (e.g., character ATK +3% instead of +2%). If you're not going to max your SSRs you might as well use maxed SRs. Comparing a LV40 SR and SSR, each with 2 ATK buffs, the SR will actually be a net damage increase. Not to mention actually good SSRs are pretty rare, while good SRs are not (I have adequate SRs like ATK+DEF buff for all 6 characters of my Wind team with just 3 months or so of farming).