There is no valid reason to use a heavy bit, whether you mean weighted snaffles or severe mouths/shanks. Corrections, twists, etc are things that shouldn't be used for any reason.
Nothing on the head creates true headset. People try to force a headset with war bonnets, brain chains, tiedowns, martingales, draw reins, on and on. None of these create headset; that comes from the horse's proper engagement of his entire body. Next time you watch a horse who has been trained and is ridden constantly in draw reins, take a look at how hollow his frame is, despite looking like he has proper headset.
Horses also have their own personal headsets. Some will never naturally and comfortably have their heads high or low, and forcing them to do so results in the rest of their body being used incorrectly.
Aids should never Be the reason why your horse is achieving a particular headset. You should have a good enough trainer and enough time put in to get your horse to do it naturally without a 4" shank hanging from its mouth. Then again, a bit is only as harsh as the hands that use it.
For me it depends on the rider and bit together. If it's a harsh bit and the tidier is pulling and wiggling their hands to get the horses head down I have a huge issue with it because it leaves the horses back hollow and strains their neck which can cause issues in the long run.
If the rider is pushing the horse forward to the contact and the horse accepts it and lowers their head while still pushing forward I have no issue.
With what Kinsey said, sadly it is true that in dressage many people have "fake frames" where the horses neck is high and arched because the person is pulling so hard and pulling back and forth, causing the back to hollow.
In many cases though people do achieve that frame without a heavy hand, that requires little to no contact or pulling if done properly. In this case the horse will stay connected. And raise their spine.