The only hesitation in letting him go for me would be the fact we've put two years into him.
I don't think he has hit the heights he should have - even when we were playing well last season. He often skips past defenders then *almost* gets the ball away but ends up losing it - it's quite strange. For me, he's still an almost-there player, not quite good enough to be taking up a precious import spot. Might be harsh; but if we want to win the league we need at least 3/4 of the imports to fire.
Imports are a lottery though - given what we know of ARod I'd much rather the Nix kept him and continued to develop his play rather than take a punt on an unknown. I think he's technically better than most other midfielders in the A-league, just has a few rough edges that should improve over time.