You're not emptying the farm. The farm is as flush as it's been in a long, long time.
Even a package around Cecchini, Swihart, Barnes, Ranaudo, and Doubront leaves you with Betts, Bradley, Bogaerts, Owens, Ball, Vazquez, Margot (breakout candidate, seriously pay attention), Workman, Webster, Marrero.
You can't just hoard all of your prospects. Half of the point of loading up your farm is to make deals for proven talents.
Look at the Adrian Gonzalez deal. Does it suck that we don't have those prospects still? I mean, really, does it?
Rey Fuentes: Never broke any top 100 lists, hasn't done anything at the MLB level.
Casey Kelly: TJS before 2013, dropping on most prospect listings.
Anthony Rizzo: Shows promise but has really fizzled in the bigs so far.
Point being, prospects fizzle out and a lot of times you do better by trading them for a proven piece than to sit and watch them fizzle.