I would really love to be able to use Position Representations with weldments. I'm sure that it's disabled based on the idea that when you weld something, it shouldn't be able to move anymore ... but there's plenty of things that, perhaps, should still be able to move.
Think, for instance, about a door. It may be welded to the hinge, and the hinge may be welded to the frame. In a drawing you may wish to show a view of the door both in an opened and a closed state, but the way that it's set up now you pretty much have to set up a series of LOD's or something to work around it, or you can make a weldment with one half of the hinge and the door, then another with the second half of the hinge and the frame, and then make a third assembly incorporating the two subassemblies.
The first approach - using LOD's - is complicated and annoying, and causes issues with changing the design later because you'll have to close the drawing to edit the part, and vice versa. (Can't edit one LOD while another is open!) The second approach does not lend itself well to doing drawings with parts lists, since the natural way of looking at things would be Door x 1, hinge x 1, frame x 1, and this would force you to have Subassembly 1 & subassembly 2, each with half of the hinge.
So, yeah, please just enable Pos Reps in weldments. Put a warning on there if you want about messing up your welds, but please just let me have Pos Reps in weldments.