"having the ghost touch special ability"
Ghost Touch does not change the base nature of the item to which it is applied.
A spectral copy is still incorporeal and may pass through solid matter.
I'm not arguing that it should target touch, only that there is a way for incorporeal weapons to offer the same benefit as material ones.
The line about "being part of the ghost's form" is not indicative of anything other than the objects' inseparability from the creature.
I would agree that the weapon, in the case of Grave Trappings, may be used either to make the normal incorporeal touch or as the object it appears to be using the Ghost Touch rules. (keep in mind, this special does not actually count toward the +10 limit of the weapon, it's quite broken in that regard)
Why bother? Incorporeal creatures have no strength score. Barring swashbucklers, dex-to-damage, etc, they're better off using their touch attack.
This is generally the case, but Conductive can really help this. Coupled with certain abilities like Sneak Attack, multiple attacks (from BAB) end up being more advantageous for a few edge builds.
Also, regardless of dex to damage, incorporeals always use dex to hit.