Rondo had been injured pretty much all season (10/11), and then his backups (West and Bradley) were also injured, which meant Nate had to handle the PG duties, along with Pierce playing point-forward.
I though Nate did an admirable job that year, but I'm fairly certain Doc hated him. When he was traded, it was more as filler because Danny wanted Green (actually, he wanted Harden) and because since we had to give up Perk (our only truly healthy center at the time) we needed a big back (turned out to be Krstic).
I do agree, though. Nate has been brilliant for Chicago this year.