What class are you?
You'd think that the trinket from FL would boost your DPS from what it used to be with your BWD trinket right? Well it doesn't look like that's the case here.
I replaced Fury of Angerforge (http://www.wowhead.com/item=59461/fury-of-angerforge) with Apparatus of Khaz'goroth (http://www.wowhead.com/item=69113/ap...-of-khazgoroth) and really haven't seen any difference in my DPS.
Of course there is a difference in that with the Apparatus, I crit more often for the duration of the effect, but in terms of viability, I don't see any reason for the 378 trinket to even exist if it doesn't provide a noticeable benefit over the 359 one.
While they are both on use trinkets, that is where the similarities stop. One gives crit, the other gives a str boost. The truth of the matter is neither trinket is that great. You'd be better of with Heart of Rage and License to Slay as fury.
http://www.wowhead.com/item=68972 is actually the one you mean if you say 378
So - lets compare (ignoring buffs, since they'll affect each roughly the same)
Apparatus is 383
FoA averages 1926/6 = 321 - but when used with other CDs (2 pc T12, DW, Reck, etc) it's effect can be magnified.
Apparatus average depends on stacks, but 5 stacks = 2540/8 = 318, but as above, the effect can be magnified.
FoA is 321
So based on the averages Apparatus is comes out slightly ahead, but if you're really good with your CD usage, I can see where FoA would be very comparable.
Maybe they've changed it, but Apparatus is also has a slight advantage in that you can almost always use it - maybe I was just unlucky for a month straight, but I never seemed to be able to use FoA when I wanted to since I didn't have 5 stacks (though this was in T11).
The difference between FoA and 378 Apparatus is fairly small too though - according to Landsoul's spreadsheet, with my gear, it's a 200 DPS difference, which is easily hidden within the varriences within fights, or even testing on dummies.
And as for why do they create trinkets that aren't clearly superior - because they try to make them unique in some way. Sometimes they are clearly superior, sometimes they aren't. Trinkets are about the only item that you can't just look at item level since so much of their value is built into the abiltiies rather than raw stats (if the trinket even has raw stats).
If trinkets were as mudane as weapons - where you have 1 accuracy trinket with a strength proc and 1 strength trinket with a crit proc or something like that, they could scale them fairly easily. But they don't do that, they do try to make the trinkets interesting. Sometimes they succeed, sometimes they don't.
An introduction into WarTanking (no longer updated as I've retired from WoW - the concepts will still be mostly accurate but the numbers no longer will be.) - http://www.tankspot.com/showthread.p...101-The-Primer
Honestly when it comes to on use trinkets, the Essence of the eternal flame is a better trinket if you are discussing fights in which you would like on demand usage. Fury and Apparatus both require you to do some form of stacking in order for the full value to be used. The crit gained from the apparatus really ends up feeling trivial a lot of the time in truth. I tried out the apparatus quite a bit due to purchasing one early in patch 4.2 but ended up realizing that in more cases than not it was a dps loss for me to use it over my heart of rage or license to slay.