Unlike many of the posters in this thread I remember when bonus loot actually gives me something. As a Prot Warrior there are 3 pieces of gear I want from Wrathion for example. Within my first three mythic Wrathion kills I had all three items and two were from bonus rolls.
The chest doesn't seem to cover garbage stat items. Which I got no end of crap from my Weekly Mythic and PVP Caches. It looks like you can get valuable currency if you don't like your drops but that still leaves the chance you'll get crap even if you play like a savage and get all of your chest chores done.