It is a complicated balance. Most prospects fail. But you also can't afford to trade them all away. In part because most of them fail!
But the Phillies didn't get any mega prospects for Cliff Lee. That was a salary dump and he was a pending free agent (albeit with a full rather than half-season and Seattle probably did better in July because there was less $ involved. And then of course the Phillies signed him in free agency!). They did give up a mega prospect in Drabek, who failed but that doesn't mean he wasn't extremely valuable at the time. Ditto the other pitcher in the Cleveland Lee deal who was younger and had known injuries.
Recent history (Sanchez. Howard, Gowdy) suggests that in the end the Phillies will be doing well if just one of the current bandied pitching trio pans out, and really well if two of them do. And they may yet trade one of them.
But probably not this year. The reality, IMO, is the Phillies are only in the 6th wild card race, and possibly with the Brewers rather than the Cardinals. They could get to 5th but they are not going to be making a run at the Mets or Braves. And the Mets, Braves and Cardinals will likely all improve more than the Phillies (on paper at the deadline I mean, Phils obviously counting on their injured guys coming back too).
The kind of moves the Phillies made in 2008, 2009 and 2010 were due to being an actual top contender, not a fringe playoff team.