There are two components to which player (abstract, not KG vs Perk) is more important. How good the players are and how good their backups are. If Delonte gets injured so Rondo's backup is Bradley and PP's backup is Green, we're much better without Paul than without Rajon. If Green gets injured so PP's backup is Wafer and Rondo's backup is Delonte then we're much better without Rajon than without Paul.
We used to fare better without Larry than without Robert because Wedman (Bird's backup) was a better player than Robey or Kite or the like.
I think this post nails it. Perk's back-up is Shaq (and, God willing, Jermaine), with Baby as further depth. Pierce and Ray going into likely match-ups with LeBron/Melo/Kobe with the pint-sized trio of West, Wafer and Bradley as their only potential back-ups.
Danny, as ballsy as he is, wasn't satisfied with a stopgap like Jamario Moon or Anthony Parker (as I was prepared to). Heck, it doesn't seem like he would have been satisfied with a healthy Marquis. He wanted a legitimate 2-way player who would add a brand new dimension to an offense that headed into the tank after Shaq got shelved, Nate imploded and Baby's shooting %s started to regress.
I'm not sure if Green is that player (he's going to have to score more efficiently to really justify the risk; wish it could have been Gerald Wallace!) but I like the line of thinking. We had 3 starting-quality centers, all injury risks and absolutely nobody of quality to back-up Pierce.
2012 certainly played a huge role as well. If we were willing to take on a contract like Hamilton or Maggette, we could have kept Perk. But Ainge is really looking to make a splash in free agency.