I disagree with you on this one Nick, trade exceptions are almost universally worthless. They just aren't used for anything the vast majority of the time. They give fans lots more options with the trade machine web apps but that is largely it.
The scenario you outline isn't even "smart", its the outcome of the Kings saying they're going to sign Green and the C's refusing to match but grabbing a trade exception for nothing (similar to what the Pacers and New Orleans did with Peja, made it a sign and trade when New Orleans could have just signed him outright). This scenario isn't truly a trade, rather its just Jeff Green leaving in FA.
I mean yeah its better to have the trade exception than nothing, but functionally the trade exception is nothing.