I can see the point behind offering a person as someone who is handy to have around to do something for you, but then again persons are a lot more than that besides their handiness. And by gods, calling Yavanna "handy" nears blasphemy!
Of the rest I'd say Formenos doesn't hit well enough. Yeah, you could say any stronghold you build is handy for you, but in the end Formenos didn't serve Fėanor & co that well.
Isildur's Bane I have hard times calling "handy". It's something akin to calling Yavanna handy. You don't call the vessel of the doom of the whole world "handy"...
One could argue that Anduril came in handy to Aragorn as a mark to prove his lineage - although having a possession of someone else's sword is not actually "The Proof" of anything. It did take part in some great deeds, but also at defeat (it broke the moment you needed it) - and Aragorn didn't seem to have made any outstanding heroics with it's help either.
The Blades of Westernesse could be handy as well, especially as it is in plural, like "these handy things one should have especially if you happen to stumble upon any wights or wraiths like the Nazgūl"...
Hmm... hard to say.
Let me leave with a kind of "take back what I said" -option. Thuringwethil. If anyone, then Sauron is able to regard others as things or utensils - and thus coming handy to him.