I finished the game a few weeks ago. The only two times I needed the pickpocket skill was to cure all three victims of the plague in Rivertown and to join the Thieves Guild in Verdistis. I had a pickpocket skill of 2 and had no problem completing either quest.

There are three victims and only two cures. You will have to either pickpocket the cure from one of the two you cure in order to cure the third OR kill one of the two people you have given cure potions to in order to cure the third victim OR let the third victim die. Allowing all three to die without the cure will not break the game.

Another way to resolve this quest is to use the dupe cheat and duplicate one of the potions and cure all three without killing anyone or allowing the third victim to die or pickpocketing the potion from a cured victim.

The only other time was when Rob asks you to steal a necklace from Sweeney for the first test required join the Thieves Guild. Sweeney is sitting in a corner of the Ducal Inn and is drunk making the pickpocket an easy quest. Not getting the necklace will stop you from joining the guild which is necessary to complete several other quests.

Having a level 2 pickpocket was all I needed for both quests. I see no reason not to have a level 3 or even a level 5 pickpocket since it is easily acquired and useful in these two situations and many other situations as well. Depending on your character build it could be essential to have the skill.

Edit: I didn't see the pickpocket requirement as being a bug but only as a skill required to advance . . . much the same as lock-picking which I needed many, many times.

Yes, it would be a required skill to join the thieves guild since killing Sweeney would probably cause dire consequences. I think it is only required as a means of teaching you to use skills. If you are playing a Survivor . . . probably a required skill for their quests . . . I was playing as a warrior and only needed it as described above.