The choice comes when you have a team that you know is going to the playoffs, and you are offered a chance to trade your top prospect for a player that will improve your team even more. At that point you are facing a complitcated choice. If you improve your team from 95 wins to 100 wins, does that actually make a material difference in your chances to get through the playoffs to the end, vs how much have you decreased your chances of getting back for another shot in 3-4 yrs? It's pretty clear that you improve your odds of winning a WS the more times you get to the playoff. The historical record is weaker when it come to trying to build a super team for one all out shot.
But even that isn't all there is to consider in that deal. The way the CBA has evolved can you even keep your team together until that prospect is ready to help you win? Are you just as well off to gut your system and go all out because there is no way to get off the boom-bust cycle MLB has set up for teams. (not saying this is necessarily true, some teams do manage to be more consistent, but it's certainly a part of what every team faces looking 5 yrs out).