Geee, thanks for bringing that trade up again. We all LOVE to think about squandering Forsberg in exchange for 1.5 years of a disgruntled Erat and an AHL center.
As best as we can tell, the thinking was the team was floating around the playoff margins and was desperately missing a top 6 LW (Ovechkin was playing RW at the time under Oates and Brooks Laich was injured/terrible). Erat still had term remaining on his contract, so he wasn't being viewed as a rental. With the team on the fringe of the playoffs, many speculate that it was a move for McPhee to attempt to save his job (missing the playoffs ended up costing him his job after the next season). It also was a 180degree pivot from McPhee's prior history of "hoarding his nuts" when it came to prospect management (historically refusing to deal away quality prospects to help his team in the short term, which had perhaps cost them in the playoffs in the past).
Most Caps fans hated the move at the time, and it's only become worse as time progressed. Erat got hurt shortly after arriving in Washington, and then Oates tried to convert him to center the following season. The apparent dysfunction between Oates and McPhee only made the move worse, as Oates never really gave Erat a chance to play in the role he was brought in for (not that Erat's boxcars gave him much reason to).