ok... how does something like the band of regen A tier then? It never really becomes useless
It kind of does - by mid-late prehardmode, there are 4 situations you should normally be in.
Melee: High defense, moderate damage. Can kill or tank pretty much any threat, maybe besides bosses.
Mage: Two possibilities. High damage and low defense, and survivability isn't something you want in a glass cannon build; The other is Meteor Armor + space gun which just shreds everything, no regen necessary.
Ranger: Ranger has moderately high damage in exchange for moderate defense - more defense than meteor armor with necro. With almost as much damage as mage, there is barely any reason to have the Band of Regen. It is possible to fit it in here, but is it really worth it?
Summoner: Unless its a challenge run, nobody is running summoner prehardmode unless they're either really good or really stupid. Either way, summoner has even lower defense than mage and basically requires that you dodge or you die. Either way, the band will not save you.
Also, there is
never a need for the band in bossfights. Almost every boss arena constructed has campfires and/or heart lanterns, negating a need for the band. Even for something like WoF where you most likely won't have enough heart lanterns to go the whole way, are you
really going to tell me you can't spare half a stack of torches for campfires?
With all this in mind, there are only two things that the band is good for after early prehardmode where you need every ounce of survivability you can get - Blocking yourself off and using it to speed up healing, not a viable option in almost any scenario due to worms and wall-piercing enemies, and the Charm of Myths, just having a bit of free healing on the Philosopher's Stone. Not only that, CoM is hardmode. The only use the band of regeneration has is as something to quickly swap in if you find a safe spot when you're low on health. It's like the panic necklace and magic cuffs in that you need to be hit for it's effect to work - because you can't regen if you're at full life - except all that effect does is slightly increase your regeneration.
With all this said, it should be evident that the Band of Regeneration sits firmly as an early prehardmode accessory, that is outclassed by the time the player reaches around the evil boss tier. Definitely not A rank material. I won't argue with the fact that in the earlygame it can be a bit useful, but it seems very overrated beyond then. By the time you can deal with Skeletron or the evil boss, you shouldn't have this in your loadout.