From a Wallabies perspective, I guess you can make some arguments for the benefits of the move. But it’s crazy talk to say we in Canberra are better off letting Melbourne get a replacement to equal (if not better than) Mafi because a young guy now gets opportunities. If Foley moved to Melbourne and Tahs fans were saying how happy they were that Mack Mason wouldn’t be stuck riding the pine anymore, we’d think they’d lost their minds.
I don't think that's really your equation though. You've lost Naisarani and signed Samu. Plenty think that Samu might be a better overall signing.
Valetini is likely to be the replacement at number 8 for Naisarani but overall it's arguable that the Brumbies have improved their backrow stocks.
Naisarani didn't take a contract at the Reds because the Force offered far more. He then took a one year contract at the Brumbies and now he's moving to the Rebels. I don't think it's been announced yet how long he has signed for.
It definitely seems like he has backed himself to be a gun for hire and is trying to maximise his earnings as he develops. It's a perfectly good strategy and so far it's worked for him and he's delivered on his contracts. It comes with risk though. If he gets injured next year I wouldn't be surprised if he never plays for the Wallabies, heads overseas and ends up representing Fiji.