Maybe that applies to the Pirates but if you look at the Royals' trades since the late 90s (specificallly, the Appier, Damon, Beltran, and Dye trades), they were all about turning pending free agents into top 100 prospects. Exactly the sort of moves a rebuilding, small market team is supposed to make, right? But it didn't work.
They sent 0.5 seasons worth of Beltran to the Mets for the #85 prospect (from the A's), Mark Teahan: 2.4 WAR, 831 games.
3 years of Jermaine Dye for Neifi Perez, ranked #33 at the time (2.6 WAR, 1403 games).
They sent a half-season Johnny Damon rental to the A's for the #15 prospect at the time, Angel Beroa (again from the A's): 1.0 WAR in 746 games.
They got 3 prospects for Appier in 1999 who combined for 3.0 WAR in 197 games.
It wasn't until Moore flipped that strategy on its ass that they finally turned things around.
edit: Correction, Beltran was traded with a 1/2 season of control left, not 1.5. Back in the days when you could get a supplemental pick for rentals.