The Faulty Countermeasures damage card reads:

"Ship - You cannot spend exhausted defense tokens"

What does that actually do? Isn't that the normal state of affairs that you cannot use exhausted defense tokens?

Normaly you can spend an exhausted token, that will result in the removal of that token from the game, never to bee used again.

As Smuggler points out, you can in fact spend an exhausted defense token, but then it's gone for good.

Mostly this prevents that last ditch attempt to survive by the target by discarding defense tokens rather than getting blowned up.