If you're running Mordred (or an equally effective affliction resist down effect) with elemental advantage, then yes, affliction rate up shouldn't be necessary. But for everyone else, affliction rate up is clearly the best effect available from accessories. It's not even close. Having your DEF down miss is soul crushing and can turn easy clears into a struggle.
I have elemental advantage on my Wind team (+70% character attack) and clears take much longer than with my Water team (~+120%). It takes my Wind team about as much time to clear Rank 3 (Thunder) as it does my Water team to clear Rank 4 (all other elements). The difference between those 2 teams is literally Cthulhu's DEF debuff.