Well, Major, they aren't quite worthless; they work for free, after all, so if you stack captured workers together they are every bit as effective as your own workers, at no upkeep cost. Many would say that this makes them more valuable than your own workers.
I rarely trade away workers at all, either captured or otherwise, and only do so when I find that offering a worker in a deal is the only way to push the AI over the line to acceptance. However, in vanill Civ III, I bought foreign workers at every opportunity, especially in the early game, because they were so cheap and they were a great opportunity to improve my infrastructure while slowing down my opponents' development. In PTW, however, the AI's valuation of workers has increased quite a bit, so it's more of a wash.