Some crates (like the Fall Crate) are supposed to be a limited time item, set to disappear at a given date and time. Yet those that acquired the crates during the event and didn't log back in the game still have these crates in their inventory, and can trade them.
Provided that they would disappear the moment you log in TF2, is there any purpose in trading them besides collecting them? Is there still a way to open an expired Crate?
Best Answer
As you mention, the special keys required to open these crates will eventually become normal keys. Therefore, they are indeed no longer usable, since they cannot be opened in any way and will disappear from your TF2 inventory the next time you start the game.
So no, there is no other purpose in trading them besides collecting them.