Glaus is having trouble running the bases. ATL cannot afford to go 15 games w/o him to give him adequate rest unless they get a power bat to replace him with.
With Glaus goes the offense. When he is slumping, Atlanta this season goes on horrible losing streaks. When he was hot between May and mid-June, they had the best record in the league.
A well-stocked farm system also enables this trade to be A OK. The Braves simply cannot afford to go without Glaus (who needs some rest) and Chipper and expect to keep up with the Phillies when batting Diaz in the cleanup hole.