Hi I am at level 39. Basicly the normal duelist are: only source of steady exp, Good source of rare Card drops to convert. Sometimes some Lucky SR or even UR Crystal Drops. Only source of Keys. BUT thats it or? Anything useful Le you fight higher lvl? Is there anything Good the Drop. Maybe Rainbow Packs at lvl xx? Or just the same?
Situational Cards that are very useful come from Standard Duelists. Such as 'Owner's Seal' for Lava Golem, 'Cup of Ace' for Deckout, and 'Jade Insect Whisle' for Weevil Burn.
Overall, unless you want to make one of the decks stated above it's quite useless and time-consuming to farm Standard Duelists.