I think it's a selfish attempt for a player to earn more money for themselves.
Where has the simple concept of loyalty gone?
Andrew Walker has to be the classic case for me:
Started off in Rugby Union.
Went to league and debuted in 93 with the Dragons.
Went to the Roosters in 95.
Went back to Union in 99 (just so he could try and play in the 99 World Cup).
After an unsuccessful time here (alcohol being the main problem), he signs with Manly for a two year, $400,000 deal.
In the middle of that contract, he is found to have taken cocaine, and is sacked.
He now runs a gardening business, and is considering an offer to go and play rugby league in France.
I think a player should just stick to one code and that's it.