My personal preference is "blah was designed for... sorry, _evolved to be suited for_ doing xyz".
It's NOT a valid term. It's incorrect. It wasn't designed, it evolved. The reason it's the way it is is that if it wasn't the way it is then it'd be a different animal- or dead. Plus no two of the birds would have been identical, so there wasn't a single design. Some would have been faster, some fatter, some a slightly greener shade than the others. The ones with the most appropriate traits would be most likely to survive and propagate, and eventually the Haast's Eagle would have stopped existing and been replaced by one (or more) other Haast-derived species.
Would people trust car reviews if the reviewer claimed that cars walked when it used wheels like any other? It's just a different verb- same as designed / evolved. Or an electronics expert saying that a voltage flows through a wire as opposed to being applied over it (and current flowing through it). A tiny distinction you might say, but it's still utterly wrong.
I guess the closest you could get would be "it's design was perfect for a killing machine"; you're saying it has a design but not that it was designed.