Use it - what's the point of hanging on to it "just in case" Lebron leave? It is exceedingly unlikely (though, sure, not impossible) that they'll land a future All-NBA talent in the mid-to-late lottery as the Nets pick is projected. Plus, second half of the season is when these tanking teams get wide-eyed at top-3 picks. Nets are going to outplay a lot of these other bottom-dwellers.
On the other hand, if you dangle that pick and get someone decent in return, you might actually get to run it back for another shot at the title. This is Cleveland... you think the fans and ownership are saying, "Hey, one is enough, let Lebron walk and we can rebuild again" ? No way. Run it back, and maybe you find a guy who is going to be an asset in the post-Lebron era when you could have an IT/Love core. The right mix of players around that is still a playoff team.