I think 343 should implement a script that can enable welded structures to unweld. For example if you want to create an exploding building you would weld the parts of the building you want to fall apart together and you would also decide what properties it had (fixed for example).
You would then create a trigger of some kind to blow it up (for this example the trigger will be a normal button) you press the trigger and that enables the unweld script and change properties script. This would make the welded pieces properties to normal (so they can fall like debris) and this would unweld them so they would split into their individual pieces, this accompanied by the explosion effects we have would make machinimas and custom games very interesting.