I think the Whetstone and the TB are designed for newer players. They’re meant to be useless if you have the “good stuff.”
I agree that Whetstone + rare weapon is weaker than a UR, for the reasons you state. It will turn your Baton of Focus (with +1 to spells) into a +1 Baton of Focus, but the +1 to spells doesn’t change. And the UR BoF is +2, so they’re not even close. You can find other examples where a rare does turn into a 2007 UR with a mere +1/+1, but I challenge anyone to turn any rare into one of this year’s three.
It’s flexible though. One will let you switch classes and you can use it with your new weapon. That’s cheaper than having a dozen backup UR weapons. But it’s not as good.
I also agree with you about the cost. Looking at the progression of price from common through legendary, and since the Whetstone feels to me like it turns a rare into at best a half-UR, and assuming this is going to be aimed at newbie-ish players, it just feels twice as expensive as it should. But there’s the flexibility aspect too, so that’s worth something.
But recipes always seem pricier than I expect, which is why this is the first year I recall ever commenting on recipes.
I’ve been focusing on other things. Does a Ranger armed with two swords and a bow, all rare, need one Whetstone or two? I guess the party card has just one number for hit/damage that applies to both hands.
If only one, that’s pretty powerful, and suddenly the recipe doesn’t seem quite so expensive.
"Ceci n'est pas une pipe" - Magritte