I don't know about welding in Poser itself, but the splitting is due to the figures themselves not being welded properly in whatever software was used to create them.  It's really strange that just random vertices are welded, and stranger still that the seams act like hard edges in ZBrush.

Whatever it is that's causing it needs fixing.  It's hardly surprising vendors aren't too enthusiastic about working content for these figures when we have to live with this stuff.  DAZ figures don't have splits or hard edges anywhere when you smooth them, and nor should the Poser figures.

Like I said, it's my guess the problem is down to the software that is used to create them, it can't be down to Poser because figures from DAZ don't have these issues even when exported from Poser.  If Teyon creates the figures, he's really the only guy who can nail the problem.