Bracers come in three varieties, greensteel (5/+1), mithril (5/+2) and adamantine (5/+3).
These are mainly leveling items, as in epics nigh every monster, and all PCs, will have +3 weapons or better.
Adamantine: This in itself almost rules out adamantine bracers out of being useful. No new character will have the coin to buy an adamantine bracer, and when they do have spare for one, they will likely not need it.
Greensteel: these are rarely seen, as +1 weapons are common even in low level enemies, these are virtually useless.
Mithril: these are more common as 5/+2 is somewhat decent and they are still affordable for newer characters, somewhere between 5,000 and 15,000 gold pieces.
The issue is that even being cheaper, they still fall flat really quickly. I do not have a comprehensive list of early enemies that have +2 weapons, but as early as Sea Bandits, some of them have +2 weapons, which are enemies around level 10.
I am almost sure it only gets worse from here, I can try to make a list of enemies with +2 weapons below level 20. These items had the potential to ease some of the leveling pains of certain builds, such as 2-handed, or two weapon fighting builds that traditionally are missing 5 AC from not wielding a shield, but as they currently stand, not so much.