While creating a sorcerer who specializes in hunting undead for a one-shot that requires us to be members of the Esoteric Order of the Palatine Eye, I initially thought it would be a good idea to select Ghostly Weapon so I could give my more martial allies the ability to bypass the damage reduction on ghosts. Unfortunately, the spell can only target non-magic weapons. Since it is a third level spell, by the time a player gets it, they're likely to have access to potency runes, which make their weapons magical and therefore ineligible for Ghostly Weapon.
Looking at the Bestiary, casting Ghostly Weapon might still be worthwhile in corner cases. The damage resistances that Ghostly Weapon bypasses range from 5 to 10, which means a d4 weapon would need a +2 rune for its average bonus damage to match the weakest resistance and a +4 rune for its average bonus damage to match the strongest.
Of course, that's a d4 weapon, which few players will use. A d6 weapon would only be 1.5 damage behind the weakest resistance with a +1 rune or 3 damage behind the strongest resistance with a +2 rune, and a d8 weapon would only be 0.5 damage behind the weakest resistance with a +1 rune and 1 damage behind the strongest with a +2 weapon.
Even if the resistances were stronger, Ghostly Weapon should still be able to affect magical weapons. It's silly to picture a spellcaster saying, "That's an undead, boy! Put away your grandfather's magic sword and draw your backup so I can give you the power to strike our foe effectively!"
Please let Ghost Touch affect magic weapons. If I'm spending a third-level spell slot to make someone else more effective, it needs to make them noticeably more effective. Especially since the spell's one-minute duration means I'll likely need to go into melee or take Reach Spell to grant its benefit.
