True, they don't need expirings, they could just let him go. I don't think Moon is something they necessarily need though. It's quite possible they'd want someone like Davis for him. And I don't know that I'd want to do that double sign and trade.
They don't *need* expirings but they do need to field a (at least) 13-man roster, right now they have 11 players:
Chalmers / Wade / Wright / Beasley / JO
Haslem
J. Anthony (RFA)
Cook
Quinn
Jones
Diawara
Well, 12 if you include Mark Blount -- your choice.
So adding TA in-a-sign and trade lets them meet the limit and gives them another expiring to try and make a blockbuster with at the deadline.
I like Moon for his defense, length and occasional 3pt shooting. We don't need him to be all-world, just a solid bench contributor.
I'd much rather have a defensive oriented guy like Moon than gunners like Murray or Stack (who is past done).