I do not believe anyone really intended to say, or give the impression that playing a theif is 'absolutely' skillless. The level of attention to detail, proper knowledge of ALL game mechanics, and the patience of a saint are needed to accomplish the task you used as an example. Anyone that says otherwise is an idiot and shouldn't be taken seriously. However in regards to PVP'ing, theif's have no place, there isnt anything they can contribute, except for being an annoyance.
I know a lot of people miss playing a thief like they use to be insurance came into play. Butt, UO now revolves around the "Item-Based" game system, before insurance, it was the opposite. Also, you have to remember one thing, online games have to be cattered to keep a certain degree of satisfaction towards the customers. How popular would a game be, if that game revolved around items, players spend tons of money and time, obtaining them, to have them snatched away.