I agree there is a lot of tanks who do not gen for aoe threat but that is why it is more important to give them a second on multi mob pulls.. cus otherwise your just gonna get eaten.
I tank with aoe in mind which is always enough to hold of single target as well but I'll watch that if I know there's a well geared mnk drg or nin in group. I think I do this cus I main blm and so tank how I know it needs to be done to hold hate against blms and so have no issues with two blm and a whm spammng aoe.
heck even in st ill grab all trash scare my healer and still hold hate against the whole raid since we only have to worry about the top hate generating dps in the grp anyways.