The Poisonbite Ring is very useful in poisonous swamps such as Blighttown and the poison pit in Lost Izalith, effectively lengthening the time players can remain unpoisoned. Poisonous enemies are somewhat rare, but the Poisonbite Ring definitely helps when fighting them. In PvP, poison is very rare. The only notable application of Poison are Poison Throwing Knives, which are uncommon and are easily cured with Purple Moss Clumps. Correspondingly, the Poisonbite Ring is not very useful in PvP, especially as there are much more useful rings out there.
As with Gargoyle weapons or Curse/Bloodbite Rings, when equipped, it looks like it creates a small build-up of Poison, but this is not the case. See Buildup for an explanation of this effect.