Lylirra has spoken. The Hellfire Ring proc was NOT originally supposed to scale off of Main Stat Weapon Damage. Also, Lylirra supports the notion that it will take a lot of farming to get a BiS Hellfire.

Direct quote (emphases mine):

It was actually a bug fix. The ring's proc damage was incorrectly scaling with a player's primary stats and weapon damage, which it was never designed to do. While we agree it was pretty sweet to see your characters do millions of damage in a single hit, no other Legendary procs scale this way -- and that's intended.

The Hellfire Ring will still be valuable, though. It may not be as valuable to players who have reached Paragon 100 now that it doesn't deal buttloads of damage when it procs, but it will still be competitive. Given the nature of how the ring is crafted, it has the opportunity to roll with some pretty incredible stats, so one of the rings you create may be BiS for you. Then again, it may not, but that's a good reason to keep farming (if that's your thing).

As for players who are leveling Paragon, we're okay with Hellfire being BiS. Yes, it will help you level up faster, but even if you stack it with Leoric's Signet and a ruby gem in your helm, reaching 100 will still take a very long time. And, as you've illustrated, the ring probably won't be a "must-have" forever, at least not for everyone depending on the stats you roll.

I.E. what this guy said:

The Hellfire Ring can still be BiS, now you just need to rolls to do so. You already have High main stat....you just need the 4 randoms to roll trifecta. It will now take a LOT of farming to get BIS hellfire ring.