Tiered plans should have tethering to begin with.
IMO it should be like here is your 2GB data plan per month. Use it as you wish, you want to tether, watch netflix, stream radio etc..
You go over then you pay for additional data if you wish to continue.
I know it should be given but we shouldnt make it sound like its a a surprising gift from AT&T to allow you to use your predetermined amount of data the way you want it.
The new mobile share data plans is at&t's and verizons new way of trying to give you less for more. And offcourse when you have a bunch of different data users on the same plan one will use up more than the others and start racking up the overages for the whole family plan.
Then give them all tethering available too and watch the profits for the carriers skyrocket

Just my thoughts on it, that's all.