The Cubs have tried to explain to the media that they aren't rebuilding, however they are to a degree. They just prefer the term builing instead of "rebuilding". To refresh the farm system with prospects, Theo and Jed need to trade players, which they see as short-term assests, at their highest point of value.
Value can be calculated in many other ways besides performance however. Some of the the ways it's calculated by are, team control, salary, and the market for that position. For example, right now would be a bad time to trade Geovany Soto since he has two years of control left. The ideal way to handle his situation would be to wait to the trade deadline and hope he's having a great year. The often talked about Matt Garza is being held on to at the moment, for this we should all be giving props to General Manager Jed Hoyer. He has held on to Garza long enough where the market for starting pitching is almost all gone, with Oswalt and Jackson the last two big arms on the market the demand for Garza will sky rocket. Especially for the main suitors such as the Red Sox and Tigers. What would be good be a good package for Garza? I over value him most likley so I will not guess. However the value for him is now much higher than it was at the begining of the offseason because of the Cubs upper staff holding on to him.