This is a bad idea, but not because one guy or another is better or worse, or older or younger, or whatever. Nash and Rondo both need ath-e-letes (to quote Barkley) to do their thing - Rondo gets bored in the regular season because his teammates and the system he runs bore him. Sometimes he'd get inspired (notably with Shaq, when there was another creative thinker on the floor with him), but less often than you'd think. Oftentimes it must feel like he's playing in the YMCA with three orthodontists and a tax lawyer, or pickup ball with his dad's friends.
Playing with cagey veterans is very useful for a young talent for a time, but now that he's a little more seasoned, I'd guess that with his imagination and creativity, it's getting a little stifling to walk the ball up all the time and wait for Ray to run around some screens or Paul to set up in isolation. And it's not just an age thing, it's an attitude - I love Pierce, but he's always had a deliberate, deliberate game. Remember those Paul and Antoine teams trying to run the fast break? It was comically ugly, the basketball equivalent of bacne. Cousy and Heinsohn muttering to themselves on the telecast as those guys barrelled down the court, bunched together like little kids on a soccer field, was hilarious.
Anyway, Nash as a shooter would help, Nash as a defender would hurt, and Nash as a facilitator would be just as stifled as Rondo is. Nash would do his best work on teams like NYC or Atlanta or Miami or the Clippers, where he could organize chaos. Not in Boston.
If the Celtics are going to trade Rondo to run the same system with the same players (which they won't do, realistically), we'd do well to trade for tough-minded ball pounders like CP3 or Deron Williams. Not Nash.