Picking up useful players that aren't wanted by their current clubs is something all good teams do. Luke Garner won the grand final with Penrith last year, for one that's close to home, and they were also happy to pick up Papali'i. Melbourne have a list as long as your arm of players other clubs discarded who have turned into good contributors. It's probably obviously not the only thing teams should do to build a roster, but finding value in unwanted players is absolutely a worthwhile thing to do.
You don't think Penrith have moved on any players they would have preferred to keep? Koroisau, Kikau, Leniu, Crichton, Luai, Fisher-Harris and Turuva were just on the discard pile?
Fundamentally, it comes down to being a better judge of players than other clubs - there's no magic formula to what sort of players you should or shouldn't pick up, other than wanting to use less cap room on them than they contribute to your performance. A player who's worth $500k is a bad signing on $800k, but if the club giving him $800k will wear half the contract suddenly he's a good signing.
