"I really dislike those detached sleeves that are so popular in Poser-dom. Is it because shoulders are hard to conform or something? I just can't imagine wearing a dress with detached sleeves. How they they stay up?"

There's a lot of complex things going on in the shoulder/collar region of Poser figures, particularly JCMs and JCMs effecting multiple simultaneous rotations. I wouldn't be surprised if some cut corners to avoid messing with that.