IF the pallie tank is good, putting dots on every mob helps a lot, because it increase the dps a lot, and it helps the tank because he will get hit a lot more.
OFc it can resume to person, so have a chat with the pallie that will gonna tank and ask his opinnion, if he can't or doesn't think is good to build agro on every mob to let you "aoe" then don't do it.
Focus is for another kind of tanks.