Really, it depends on your point of view.
From the point of view of an average ghost, there's no point in having a toughness score or a reflex score. Insubstantial 4 protects them from the average NPC they'll encounter. This due in equal parts to the average NPC being incapable of adding Affects Insubstantial to their attacks with a power stunt, and both the ghost and the average NPC being probably unaware of what common effect still works on the ghost. And even if they encounter and do combat with other ghosts, it's not like they can die again. (Really, the difference between a person with Insubstantial 4 and a ghost is more than just descriptors in my opinion; a ghost needs to have some ranks in Resurrection) Really, all they need is a good will score to protect themselves from would-be exorcists.
But consider it again from a point of view that contains knowledge some would consider meta-gaming. Like FuzzyBoots pointed out, any PC can add Affects Insubstantial to their attacks with a power stunt and a good descriptor. There are also (though they like to keep their presence hidden and existence unknown) beings who have attacks with Affects Insubstantial by default: angels, demons, devils and mortal magic users. So for the ghost of Joe Schmoe, toughness and reflex scores are pointless. Most of the time it won't make a difference to him, and he's already screwed when something with a hellfire attack or enough spiritual prowess to strike ghosts with their bare fists shows up. But for a PC ghost, or a ghost that you intend to use as a supervillain, toughness and reflex definitely have their uses.