Couple things here.
1. I think the Nets are okay with an expensive one year rental at PF. They were heading towards a 4/5 rotation of Favors (Mr. Upside Project), Lopez (really good) and Petro (really not good), and Humphries (selfish offensive player). Yeah, they could keep planning for the future, but there's still an attempt to draw fans and play 82 real games; Murphy helps tremendously in this regard. They have a real roster now. It would suck for players and fans to run Humphries, Petro, Favors, and Lopez into the ground; not good for Favors' development either.
2. Outlaw and Morrow are nice pieces, who, along with williams and James, make Lee expendable. They were able to help their frontcourt this year by giving up an expendable piece; fine.
3. They will have max capspace next season after renting Murphy for one year; Murphy doesn't change that at all.
Harris/Morrow/Carmelo/Favors/Lopez with Farmar, Outlaw, James, Williams off the bench is not a bad foundation.
4. Financially, their new owner is insanely wealthy, and has no problem paying Murphy as a placeholder for one year while Favors develops and they can try to make an impact next offseason, so it's really not a waste of money.