I have a lot of experiance doing this, so Ill weigh in here.
Your job as MT is to make everyone beating on you. Ideally, this means that you wont hav eto move however thats not always possible. You should be attacking every mob in a given pull to spread your aggro out.
In an average pull for instance :
Bloodrage
Shield block / revenge target one, hit a few times, maybe shield bash.
When revenge lights up again, revenge mob two, then start hitting mob 3. Shield bash mob 2.
If mob 1 gets away from you due to DPS you then taunt that off and back to you (only use taunt to regain aggro -never to build it).
The reason MA doesnt peel is because then the DPS would move to that target. You want the MA focused on one target till it dies, no exception (This is to keep all the dps constant and not helter skelter). As MT your target will likely be changing every 4 seconds in a multi pull.
This goes out the window in MC Type places of course, where basically you tank A mob and hope you dont die