You're probably right; I broke one once, and in hindsight, it probably wouldn't have broken if we'd bothered to chase the BB threads. I could see how over-tightening would cause problems, too. I still try to avoid plastic on my bikes wherever possible. While it's true that it cannot corrode, it absolutely can fail in about 100 other ways. I guess it might not, in the case of a BB cup, but I know we've all seen busted plastic cups on new acquisitions or other ppl's rides...

-rob