I've been thinking about it for a while, and I'm not sure that the Abyss Shard is the optimal choice of reward from the warlock quest after all.
The trinket's benefit is that it saves you a soul shard and 60% to 100% of your base mana, depending on the choices you made for talents. But, by taking up an additional slot in your bag, the benefit of eliminating the soul shard requirement is negligible. There should almost never be a situation where you go a half hour without getting access to a new soul shard, unless you've planned very poorly. And, is depriving yourself of another trinket to use in some given fight really any better than saving that 100% of base mana, when you could be using the lengthy cast time to cast other spells instead?
Untalented, you benefit more from the free mana cost, but have to put in additional time casting. In the time it takes to summon your pet, you could be casting two mana drains instead. Talented, you've got an easier time casting quickly, but are only getting a one time 60% of base mana cost reduction, which seems much worse than using a mana trinket over the duration of the fight.
People say that the trinket is irreplaceable, but from where I'm sitting that doesn't seem at all true. I admit that the trinket has unique utility starting from an emergency situation where you're out of mana and can't afford to wait for it before you need to summon your pet, but with a lengthy cast time such situations are going to be rare, and you'll probably avoid getting into such situations in the first place if you pick a different trinket and cast different spells instead.
However, I am new to the class and have no experience with it, so maybe this is all me being an idiot. What are your thoughts?