This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
I suppose the answer is that by not having to redo the query, you save time. However, this is dependent on the scenario and involves a memory tradeoff. So it depends on whether it will increase performance.