The main reasons to why Warlocks don't use Infernals I've heard are the following:
1. It can be easily banished by an enemy warlock.
Fair enough.
2. It can be enslaved by an enemy warlock and turned against you.
Makes sense.
3. The aura-fire damage breaks CC in groups/raids.
Same as the Searing totem, a valid claim.
4. With a one hour cooldown and the cost of a reagent, it's more a gimmick spell, or one hardly thought about. I hardly think about the spell, and it doesn't really matter too much since I can't use it that often anyway. It's a hassle to set up a Curse of Shadows and re-Enslave, and it's a liability in case it breaks early.
I can understand this. All four of these sit well with me.
Now, what happens if the conditions aren't met?
What if you see that there are no enemy warlocks in your BG, and you hovered your mouse over leftover Infernal Stones in your bag that you completely forgot about?
What if this is a bossfight where you need as much DPS as you can, there's no CC needed, the fire mages are keeping up the 5-scorch debuff?
Are there any reasons I forgot about to why I've never actually seen a warlock use Inferno, ever, in my near 3 years of WoW?