Ultimately, all of this stuff is up to the veto or go-ahead of whichever gm is presiding at whatever table is being played at.
But I think a +2 to swords as a supernatural power inherited as a scion of a war god is underpowered for a power, and overpowered for a stunt, so it defaults to a power.
Consider: Claws is -1 refresh, but gives a +2 weapon fists attack /any time/. The book mentions that weapon abilities that require weapons - items that can be lost, stolen, broken, left behind, etc should be more powerful than fists stunts.
I think that -1 refresh for a +2 weapons skill for swords is balanced, because it only works for swords, doesn't count as a stunt (so has to be a supernatural character) and it can /easily/ be taken away from a character.
In fact, an opponent with any weapon that is 4 shifts or higher, if they were to state they are directly attacking the sword as a maneuver and then invoke it for effect, I'd rule that sword broken.
::shrug::
Compared to wizards at submerged level being able to throw around 30 shifts of power in a myriad of ways, I just don't see a point in quibbling over weapon stunts/powers.
I personally think some of the most nasty powers in the game are those that affect the mind.
I have a PC in my submerged campaign who has a stress track 10 boxes long. He is very hard to hurt physically. But against a wizard firing a lance of mind-melt, or a BCV doing the brain voodoo, he will be like putty.